I have a friend, who, every time we talk, makes me feel better about myself.  I have no idea why this happens, but I know that it is so.  I have other friends that I like a lot but who do not have that effect.

Here are the characteristics that seem to define Friends of Happiness:

1. They like you for you.  Mostly, that means you are comfortable enough around them that you let them see who you [really] are.  Which means...
2. They are non-judgmental about you.  Of course they could point stuff out about you, and be judgmental about others; those make for interesting conversations :)
3. They are comfortable about themselves.  Sometimes friends see your issues as reflections of their issues, and conversations about you are actually about them.
4. They teach you about yourself.  Either by observing you or by giving you the freedom to observe yourself.

Sometimes when I'm in an introspective mood, like now, I consider "my friend collection".  Friends of Happiness are particularly valuable!