Here, I've seen genuine relationships develop--not based on looks and fame but more on intellect and genuine happiness.
Living in SF, I could let my eyebrows grow out, wear less makeup, put on a couple of pounds, and wear North Face jackets everywhere, and I'd still get asked out (doing any of this in LA might imply that I was homeless). The weather is colder, which is conducive to snuggling up.
I've dated and been single in both cities; however, I met my husband in San Francisco (actually, online). People date each other for a reason--because they want something from you, be it money, drugs, fame, sex, or status.
Then again, dating in LA can be fun because there are no feelings involved.
Being single in LA can also be fun because it's a very self-centered subculture.
People are more adventurous and since you're dealing with a more attractive population, you have to work harder to look better, which can do wonders for your self-esteem (of course, it can also have the opposite effect if you waste your time trying to compare yourself to women who are supernatural).
I've never dressed, looked, or felt better than when I lived in LA.
Also, I never felt bad about being single in LA because I could tell that 99% of relationships around me were also fake.
I actually felt like I was the only one living in reality and being true to myself by being by myself and making myself happy.
Being single in SF, on the other hand, is a little more difficult in terms of jealousy over others' relationships.
Conversations tend to be more intellectual, which is a nice contrast to the superficial LA vernacular.
In general, it has been my experience that people in SF are more committed while people in LA are more unreliable.
It's almost as if people in LA are perpetually waiting for the next best thing to come along. I was born and raised in LA but I went to UC Berkeley and worked primarily in SF aside from a 10-month stint in LA during my 20s before I wised up and high-tailed it back to the Bay Area in time to meet the man of my dreams.