First off, I've never been a fan of facial hair. Mustaches just look ... too seventies, and it itches when you kiss. I think some guys just don't like to shave, but hell, I've been shaving my legs for thirty-odd years and it hasn't killed me yet. I do think, though, that there are three main reasons a guy grows a beard.
1. He's a total stone-cold fox and grows a scruffy beard to knock you out with his now-foxier-than-ever self.
2. He's getting bald and is desperate to grow hair someplace, ANYPLACE.
3. He's getting fat and is trying to conceal a double chin.
Sadly, it's almost always 2 or 3, and almost never 1.