Why don't people take themselves seriously?
Where is the fun in taking yourself seriously all the time? People that meet me likely think I am one of the most frivolous and goofy people they've ever met but I can't say that any of them have said or thought that I have self-esteem issues. Perhaps it's because I don't.
If you want to be the dour person that no one wants to hang with because you are always taking yourself seriously more power to you. I prefer to "always look on the bright side of life!" (insert correct whistling tune here). Because when life sucks balls and everyone is against you and you don't know how the hell you are supposed to freaking function, taking yourself seriously is probably the worst thing you can do. No matter how shitty your life is, there is always something you can laugh about in yourself and laughter helps to free your positive thoughts.