Of course, the number one reason - it is an opportunity to win a particular bet.All hope to hit the jackpot, but even a small amount is enough to people continued to play. On a psychological level, the phenomenon of hope / expectations. Even one win (and thus a lot of losses) - this is a positive impetus for the person to continue playing.
During the game, the adrenaline produced, the body gets used to such a regime. It already contributes to improvement of the general emotional background, plus quite strongly developed analytical thinking.)
The most interesting thing, when you watch the professionals as they play: bets on the preflop, on the flop, turn, river, when there is the opportunity to play with the professionals and what they learn!
I've always said, make money. Although apart from this I like try to read all the hands that I play, not only to be sure that I have the winning hand, but also to know the cards with which my opponent calls.
For me, the game is sheer entertainment. I prefer playing live versus online because I can meet people and socialize with them.
Also, there is something about the sounds in a Casino that get me energized. Slot machines, hearing people's reactions to wins, chips being shuffled at the table ... heaven!