I guess the difference is that your mental context is so much more developed when you're thinking about yourself (not in the 3rd person). You have skin in the game, maybe it's that you're worried about the outcome of the situation, or it's just stressing you out, or you're thinking about how it will affect your friends/family/the rest of your life. So when you view it from an outsider's perspective, and throw away all that mental "noise", you have a clearer head and can make better decisions with the long-term in mind.
Alternatively, if you're going through something, you could try imagining someone else in the same situation (make up someone if you have to) -- What advice would you give to them? Then use that advice for yourself.