Why are there holes in my shirts?

Recently I’ve noticed at least two T-shirts I own have very small holes in them. One right by the collar, the other in the middle of the shirt. One shirt is old and has been worn many times, one shirt I just bought and I’ve only worn about three times.

My roommate swears that he hasn’t noticed holes in his shirts; we use the same laundry machines and detergent.

What is usually the culprit behind small holes in clothing?