For his own personal ego. He believed he was destined to be a great man. Life wasn't living up to that expectation. He knew by shooting the President he would be someone important.
Makes me think of the Peter Gabriel song, “Family Snapshot”.
http://www.youtube.com/watch?v=Uh9wIVZBhT0
bttt
Now I don't get that at all. If a man was so anxious to make himself important he behaved exactly opposite after the assassination occurred. He allegedly snuck out of the building, killed a cop and then got subdued in a theater. He had a black eye the next day when he was murdered by Ruby during his transfer and all along he denied doing it and complained bitterly about his rights.
Someone looking to be so important would more likely take credit for the murder.
Examining other presidential assassinations and attempts, I believe all of them were cases where the shooter was identified (Booth) if not outright caught immediately: (Fromme, Hinckley, Guiteau and many others) Hinckley trying to impress Jodi Foster fessed up, right up front from the very beginning and it would seem to me that Oswald was even more desperate for attention by all accounts of people making the argument that this was strictly all about Oswald's ego and that's all there is to it.
Check this out:
http://en.wikipedia.org/wiki/List_of_United_States_presidential_assassination_attempts_and_plots
No, not buying it. That dog just won't hunt!