If it's you, know why? 

1. Those that are in a relationship, let's face it, don't have anything in mind but their present situation.

2. Those who are in relationship are way too emotional or, let us say this they are way too in love that they actually don't care with what's going on around .. even to you, friend! 

3. The single friend is the one who has a lot of time to spend talking with you.

4. Those in a relationship is the one who actually needs advice, so what's left is you, the single friend who is the only one in her proper mindset.

5. The single friend is the one who sees everything, and dare not to be like the one who seeks the advice! LOL

So, better not be upset if you're the single friend. :)