The attraction and fresh excitement of a new lover, validation of one's ability to attract, the need for clandestine adventure, excitement added to a life perceived as boring, keeping to a macho cultural norm (as in some Italian families where the father is expected to have at least one mistress), someone to have sexual intimacy with while one's mate is long term unavailable (in the army, in a coma, in an institution, pregnant, indifferent, having his or her own affair etc), the lure of adolescent fantasies no longer part of the marriage, kinky sex interests never confided to spouse, gay and wanting sex with same gender unbeknownst to spouse, ethically immature and doesn't take promises or vows seriously, feeling disconnected with spouse but not wanting a divorce because of children or established social status/stable life, thinking of getting a divorce but needs a "transition person" to assure him/her that he/she can make it out there....
OK, I got tired and stopped.
But, the truth is, why wouldn't people cheat? When you think about it, it's pretty amazing that anyone actually stays faithful!