What keep you playing the game?

Aside from research, PoGO is a good motivator for me to for keep up with my daily exercise.

Number 1 reason for me is to complete my pokedex. I'm going to Asia soon so I'm hoping to catch my first Farfetch'd.

Also my goal is to have as many perfect IV Pokemon with the best move sets and then power up my favourites. I also like to walk and hatch my eggs I think it's the funniest part of the game.

I almost complete the pokedex (without regionals), just Charizard and Kabutops left. That's one motivation.
Another motivation is to create a dream team, I have 7 mons for attack with top IV's, best moveset and full powered up by now, and I am finishing building my defense dream team.
And of course I like to fight on gyms, just for fun not for coins, because it's too difficult trying to stay on the gyms in the city.
But the most important thing is that thanks to pokémon my exercise is not boring anymore. I am a runner and I love to run now more than ever.

Im close to completing my dex the only thing that'll prevent that is the region exclusive pokemon and of course the other 6 pokemon not available yet. Ive also been just trying to get good pokemon with good ivs and move sets even though I don't battle any gyms.

I play to wind down and get away from the noise and bustle, I go to a quiet park usually. I walk a bit further that I would otherwise which is a bonus as I need more exercise.
Sometimes I worry that I'm not paying attention to the beautiful natural sights around me (like the autumn leaves right now!) but I like the little challenges and hatching eggs is always fun.
If I can build a good team of defenders and attackers that will be a bonus.

I don't have the opportunity to play real games anymore. I walk a lot, why not play a walking game. I like the perfect IVs as well.

I think it really depends. For me its the dream team with the best move sets. People that just want to complete the dex is temporary. You should focus on team and battling if you want longevity in the game from my experience with pokemon go. I have already completed my regional dex and I still play to power up my snorlax and lapras which I still need to level up. If I didnt have my team, I feel I would have given up already. (Snorlax with hyper beam and lapras with FB and blizzard)

Honestly, I want prove to myself that I am better than others. When I get the feeling that I am the best around I'll stop playing. I am super competitive so I need rivals to keep going and improving.
There are still players I recognize as my rivals, so I am still playing.

Seems like the worst reason you ever heard? Maybe it is, but I prefer honest answers.

just to kill time, walking everywhere, look around the park, play with kitties, i call it a day

Pretty much pwning newbs, being better than everyone else. Pretty much the same reason you play any game. The fact that they gotta confront you on the streets and not in their moms basement makes is pretty cool.