Athenian Democracy - How were the citizens motivated to be so politically active?

First of all, Athens was a "city state," that is one where "everyone" (of consequence) knew everyone else. Imagine a smaller version of "New York City" as opposed to a whole country.

Second, only a fraction of people (free males with enough property to arm themselvs) could vote. That is about 20% of the population. Because they represented a "subgroup" within a group, they were motivated to maintain this status against the other 80%.

Third, Athens was periodically threatened by powerful enemies; Sparta among other Greeks, the Persians among "foreigners." When people are living "under the gun" (almost literally), it is a powerful motivator for them to "do things."

A further clarification of the issue would also be to remember that a lot of Athenian citizens actually did not live in the city of Athens itself, but in the smaller cities spread out over Attica. Many of them were situated so far from the city that they did not participate in the day-to-day politics of the state at all.

Also, I guess we should ask ourselves what it means to be politically active. If we just mean turning out to vote on issues at the Pnyx, then yes, the Athenian citizens were politically active, at least those living in the main city. However, the actual political "craft" was undertaken by a select elite of generally wealthy citizens. For example, while all citizens in theory could voice their opinion during the assembly meeting, the reality was that those that spoke were usually a part of the semi-professional group of "politicians", with the wealth and rhetorical training needed for a long and sustained political career.

Not that these individuals ruled the city (even Pericles experienced defeats in the assembly from time to time).

So, I guess it all depends on what we mean by being "politically active". Also, comparing Athenian direct democracy to our own representative democracy is in my opinion comparing apples and oranges. But that is a whole other discussion.

I believe this question puts the cart before the horse.

It is not that (aspects of) Athenian democracy somehow motivated its citizens to great political activity; but that the highly motivated political activity of Athenian citizens created and sustained Athens' democracy.

A people always get the government they deserve, and the citizens of ancient Athens earned their democracy through their own participation in its political decision making process: day by day; week by week; and year by year.

Michael's answer is a very good one but I'd like to add a couple of details. First, Athenian citizens were not always as enthusiastic about voting as you'd think: voting required a whole day which meant they'd be missing out on one day of revenue from labor. To compensate for this, Athenians were paid (type f3 and search for paid) about as much as the daily wage of a craftsman (IIRC). Despite this the assembly would sometimes end up with to few to vote;

They met every ten days in a small auditorium called the Pnyx. Many polices were debated in the ekkiesia, the most frequently debated polices were regarding money from public taxes where to go, declarations of war, and/or the signing of treaties. These were decided on by a vote, requiring 6,000 voters be present for a vote to take place. If there were less than 6,000 male voters present then slaves would be sent out with a rope soaked in red paint, to gather the late comers. They would wrap the rope soaked in paint around the late comers. Having red paint on your clothes was considered disgraceful and in addition, those individuals would be subject to a fine.

To summarize: I think your view of Athenian democracy might be too positive, I see no reason why Athenians would be any more enthusiastic than modern Americans.