You vote because you can, feel entitled to, feel responsibility to, or have an actual strong preference for party or candidate.
I voted for Obama because I believed in his sincerity.
I think I will do so again because his sincerity hasn't changed, his execution has focused on things he pledged to work on, he has repaired a ton of damage done by Bush Jr. administration, the work isn't completed, and there isn't an adequate challenger party or candidate. I also don't want any part of the current Tea Party vocal rule republican party in office... they are too radical in my view.