10) Hopes his beard will draw attention away from his constant silly
9) He and Worf are having a beard-growing contest.
8) It helps hide the faint, hereditary birthmark on his chin that proves he's a werewolf.
7) Hopes to look scarier to hostile aliens.
6) Starfleet has bridge personnel hair quotas and he's trying to make up for Picard.
5) He thinks Troi finds it sexy.
4) (seasons 2 to mid-4 only) Didn't trust himself to pick up a can of shaving cream because the temptation to go and spray it at Wesley was too great.
3) Wants to look more like his hero, Ming the Merciless.
2) Every time he does, Q makes his beard grow back in five minutes as a gag.
1) Picard won't let him fire up his photon blade.