I don't typically have much spending money, so when I get a game it's something I expect to have a great personal investment in. I think it's quite easy to find good games that are free, or to just play fun games I already own. But when I feel like I can become attached to a game is when I'll make a purchase. Attachment can present itself in many ways, be it socially in a co-op game, emotionally in a choice or story-heavy game, sheer enjoyment in the feel of a title's gameplay, etc. Even feeling connected to a developer and their design philosophy or simply who they are can be incentive enough :)