Why do I try and poke holes in everything?

I am an introvert 17 year old male, with personal interests such as music, guitar, football etc. I have a close circle of friends, whom I began to hang around with after years of ociating with faceless extroverts. Though my friends are all generally nice, understanding people I cannot help but in my mind, point out their bad points, even if they are very minor. This is the same for everything in my life, i poke holes in everything that has the potential to make me happy. If I think, something like " Im going out on Saturday", I then think something along the lines of " Yeah, but tomorrow is Sunday, and I have school the day after, so I can't enjoy myself", something like that. I can't go into too much detail, but I just have a dark outlook on the world, I'm not emo or anything, far from it, I just can't seem to allow myself to enjoy life or anything, and I'm worried that this outlook will leave me 10 years from now friendless, alone, and jobless. Any help?