What is Unwise Consideration?

I have heard that on one occasion a certain monk was dwelling among the Kosalans in a forest thicket. Now at that time, he spent the day's abiding thinking evil, unskillful thoughts: i.e., thoughts of sensuality, thoughts of ill will, thoughts of doing harm.

Then the devata inhabiting the forest thicket, feeling sympathy for the monk, desiring his benefit, desiring to bring him to his senses, approached him and addressed him with this verse:

From inappropriate attention
you're being chewed by your thoughts.
Relinquishing what's inappropriate,
contemplate
appropriately.

Keeping your mind on the Teacher,
the Dhamma, the Sangha, your virtues,
you will arrive at
joy,
rapture,
pleasure
without doubt.

Then, saturated
with joy,
you will put an end
to suffering & stress.

The monk, chastened by the devata, came to his senses.

Ayoniso-manasikara Sutta: Inappropriate Attention (SN 9.11)

And below this lack of mindfulness and complete awareness lies unsystematic reflection (ayoniso manasikara). The books say unsystematic reflection is reflection that is off the right course; that is, taking the impermanent as permanent, the painful as pleasure, the soulless as a soul, the bad as good. The constant rolling-on that is samsara, is rooted in unsystematic thinking. When unsystematic thinking increases it fulfils two things: nescience and lust for becoming. Ignorance being present, the origination of the entire mass of suffering comes to be. Thus a person who is a shallow thinker, like a ship drifting at the wind's will, like a herd of cattle swept into the whirl pools of a river, like an ox yoked to a wheel-contraption, goes on revolving in the cycle of existence, samsara.

from The Seven Factors of Enlightenment by Piyadassi Thera

SN 44.55 states yoniso manasikara is a condition for Right View.

Sūriyassa, bhikkhave, udayato etaṃ pubbaṅgamaṃ etaṃ pubbanimittaṃ, yadidaṃ — aruṇuggaṃ; evameva kho bhikkhuno ariyassa aṭṭhaṅgikassa maggassa uppādāyauppāda etaṃ pubbaṅgamaṃ etaṃ pubbanimittaṃ, yadidaṃ — yoniso­ma­nasikā­ra­sam­padā.

Monks, before the sun rises, there is the forerunner & precedence of dawn; in the same manner for the arising of the Noble Eightfold Path to the monk, the forerunner & precedence is wise thinking.

Yes, it's what you attend to, you are correct. Sometimes thinking about the future is useful though, as in forecasting results of your action before you do it. But generally speaking yes, one can make oneself miserable by focusing on the wrong things, or once can make oneself strong. The effort is the same, it's what you focus on.

If you let your attention go to things that cause craving, your mind can get infected by desire.

If you focus on the negative side of things, you will maintain yourself in wounded and miserable condition.

If you focus on good things, you can learn to enter and stay in first jhana.

MN 2 lists examples of unwise attention:

What are the things unfit for attention that he attends to? They are things such that when he attends to them, the unarisen taint of sensual desire arises in him and the arisen taint of sensual desire increases, the unarisen taint of being arises in him and the arisen taint of being increases, the unarisen taint of ignorance arises in him and the arisen taint of ignorance increases. These are the things unfit for attention that he attends to. MN 2

MN 19 provides a clear example about how wise reflection/attention is used to develop the path.

As I abided thus, diligent, ardent and resolute, a thought of sensual desire arose in me. I understood thus: ‘This thought of sensual desire has arisen in me. This leads to my own affliction, to others’ affliction and to the affliction of both; it obstructs wisdom, causes difficulties and leads away from Nibbāna.’

When I considered: ‘This leads to my own affliction,’ it subsided in me; when I considered: ‘This leads to others’ affliction,’ it subsided in me; when I considered: ‘This leads to the affliction of both,’ it subsided in me; when I considered: ‘This obstructs wisdom, causes difficulties, and leads away from Nibbāna,’ it subsided in me. Whenever a thought of sensual desire arose in me, I abandoned it, removed it, did away with it.

In short:

• having Vipallasa (see: Vipallasa Sutta),
  • I.e., not seeing the three marks of existence
• having the unwholesome roots
  • having craving and clinging due to the unwholesome roots arising
  • having defilements
• having the 5 mental hindrances
• having ignorance
  • not aware of causality (4 Noble Truths, Dependent Origination, Conditional Relations) due to ignorance (not knowing the 4 Noble Truths, Dependent Origination, Conditional Relations)
  • because ignorance also displaces the rights view no knowledge of the right view (not knowing or wrong understanding items mentioned in Samma Ditthi Sutta) or having know the wrong view.

Piya Tan has done a good detail explanation which I have quoted below:

Ayoniso manasikāra, as such, means “not directing the attention to the roots of things” or “directing the attention away from the roots of things,” that is, not observing phenomena as they truly are, not noticing that they are impermanent, unsatisfactory and not-self. As a result, wrong view arises, and when this become a habit, wrong view is reinforced so that it remains as a latent tendency (anusaya). Let us examine a little more as to how this happens.

“Unwise attention,” according to the Vibhaṅga, is the regarding of what is impermanent as being permanent, what is painful as being pleasurable, what is not self as self, what is foul as beautiful. 6 The Majjhima Commentary says these are the four “perversions” (vipallāsa). It explains unwise attention (ayoniso manasikāra) as attention that is unskillful in means (anupāya,manasikāra), attention shown the wrong way (uppatha,manasikāra), that is, by way of the four perversions, namely:

what is impermanent is taken to be permanent (anicce niccan ti);

what is painful is taken to be pleasurable (dukkhe sukhan ti);

what is not self is taken to be self (anattāni attan ti); and

what is impure is taken to be pure (asubhe subhan ti).

The (Akusala,mūla) Añña,titthiya Sutta (A 3.68) says that the three unwholesome roots (akusala mūla) of lust, hate and delusion arise through unwise attention. Lust arises and grows through unwisely attending to a “beautiful sign,” that is, being captivated by what we perceive as attractive in a thing. Hate arises and grows through unwisely attending to a “repulsive sign,” that is, what we take to be unattractive. And delusion arises and grows through unwise attention itself. In other words, delusion is present when there is lust or hate. Delusion arises and grows because of the four perversions mentioned above.

On a deeper mental level, as explained in the (Āhāra) Kāya Sutta (S 46.2), unwise attention gives rise to the five mental hindrances—sensual desire, ill will, sloth and torpor, restlessness and remorse, and doubt—which prevent us from attaining samadhi, that is, meditative mental focus, or any kind of wholesome attention. In short, we are hindered from our mental development.

Unwise attention, in other words, is based on ignorance, which starts off the dependent arising of mental formations (saṅkhāra), which are karmic activities, and these lead on to a whole mass of suffering,10 and the prolonging of the samsaric cycle (vaṭṭa) (MA 1:64 f). In short, unwise attention is the root of samsaric existence. Unwise attention is food for a lack of mindfulness and full awareness, which in turn increase ignorance and craving.

Introduction to Yoniso Manasikāra Sampadā Sutta by Piya Tan. The next section discusses what is wise attention.

Also see my other explanation in these answers.