Because you have to put up with so much, and you are working for individuals who are worth so much, the money can actually be very good. Hook up with the right celeb, and you could be making over $100k a year easy. There are some people who would pay that much just to be within five feet of Jennifer Aniston, and you now take it for granted.
Aside from the money, one of the most attractive elements of this job is the perks. Movie premieres, first class travel, stays in amazing 5-star hotels, food at the finest restaurants, the opportunity to use your celeb's house as your own when he's out of town ("Oh, yeah…sure this is my place…"), the chance to hobnob with some of your favorite stars…the lifestyle is great, until and unless you burn out on it. Seeing Meryl Streep in person is only exciting so many times.
Meryl would not appreciate that.
Of course, there are lesser celebs in Hollywood as well, and there are therefore lesser PAs, too. In general, the bigger the name, the bigger the ego, the more money you make, and the more you have to put up with. So pick your poison. There's plenty of it to go around.
Then ask yourself: "Will I still have this job at age 50? No? Then what do I do to pay my rent…?"