Getting upset at others happiness or finding your happiness in others (both of which I'm guilty of) are major insecurity signs. It shouldn't be that way yet I understand why it feels so natural to many depressive people.

It's also always nice to remember that people put on faces in public or even just around others. They don't blurt out all their problems and such. So even if they don't show it it's probably there. Few people live without problems or sadness.