maybe it's a really lousy stalker..?
there's your answer!
Because they want to increase their number of fb friends I guess, and sometimes by suggestions and mutual friends, or they might see something in common and want to actually make a new friend.
Then want make it seem like they have the biggest friend list.
to increase their list of friends , i'm guessing
that happens to me all the time on this thing and facebook usually just for random frnds they will nvr talk to. Or because u r attractive, or both.