-opportunity (environment, time, way(s) to hide affair)
-a "need" for intimacy (sexual, emotional, etc.)
-a "need" to feel desired & wanted
-a loss of connection in marriage relationship & inability to effectively communicate with spouse
-another willful partner (possibly looking for or in "need" of similar fulfillment)
Yes, those are all valid contributoers, but would more accurately for into a category of felt needs rather than circumstantial conditions.