As quoted by me to a friend...

[robinbloke]

The standard rule of relationships is as follows :-

You only ever find anyone you like attracted to you when you are already in a relationship.
These chances are doubled if it is a committed relationship and tripled if you are married.

Comments

and...

[k-lioness.livejournal.com]

Not to mention the fact that people who are in relationships are generally more confident and secure in themselves as people, which makes them more attractive to others and more open to the opportunities. People who are not in relationships (for whatever reason) tend to be less sure of themselves when meeting new people.
If you are in a relationship and the new person doesn't like you, who cares, you're in a relationship... there's no pressure. If you're single, you're more likely to feel pressure to like you, making you less likable.