>>16406566insecurity on your part because you want to enjoy yourself but probably deep down feel you can't for some certain reason. it is most likely related to why they're enjoying themselves to and perhaps you have unconsciously blocked off a certain aspect of yourself and now feel jealousy when you see others able to express that part of themself and enjoy themselves. usually this stems from our upbringing. best way is to address what specifically you are upset about and being honest with yourself, then maybe seeing a counselor for healing.
you won't be upset at other people anymore and will probably learn more about yourself.