I wish my answer was only "because I'm hungry".
I'm afraid - though less so of late - my reasons include:
because I'm bored
because it'll finish off the pack
because it's "time to eat" (lunch-time / tea-time)
to be social - join in the meal
because it's there
to be polite - I didn't like to say no to the offer
out of curiosity - (I wonder what those cakes are like?)
saw a food cue (e.g. sweets in a shop / chip shop / whatever) thought food and ate
to take a break from work or other activity - (time for a cuppa and a snack and I'll do the rest of this job a bit later)
Anyone got any others?