"Let's set aside mental issues and pathologies and deal with 'normal' people reasons. My top two reasons are, first, you aren't getting the things you want out of a sexual relationship and you go elsewhere. Or, secondly, you get bored with your present partner. If people aren't getting the quality of sex or frequency [they desire], they'll supplement on the side, but will still love their partners and want to stay with them." -- Eric Amaranth, sex life coach
Being attracted to other people is normal, but...
"It's natural for humans to be attracted to others -- whether a physical or emotional attraction, or both -- but in a monogamous relationship, individuals must recognize they feel an attraction, but overcome any desires, as adultery is a sin." -- Rabbi Jason Miller, rabbi
"Sexual feelings toward attractive others means you are still alive. You haven't died yet. At the same time, sexual feelings toward someone else can be a helpful gong that goes off to tell you its time to reinvest in your loved one. The stronger your relationship, the stronger your immunity will be to straying." -- Dr. Susan Heitler, clinical psychologist