I always thought of it as depressed people quit/happy people achieve. It makes more sense to me in terms of a causal relationship, since a happy person would be more well equipped to do things, and doing things doesn't necessarily make you happy as mentioned above. Regardless, there's a reason why you quit, and if it's something that affects all aspects of your life rather than those individual things not being for you, then I think the best thing to do is to try and fix that before you launch yourself into new things, if you're still semi-interested in trying to do things, that is. Some people might feel nothing in life is for them, so it's pretty hard to determine if there's a state-of-mind that allows you to enjoy and keep at things that is still within the realm of possibility for you, but if you feel that it's worth it, then it could be worth looking into certain issues, perhaps something that's affecting your biology or a psychological issue for example, in order to make you more generally motivated. Either way, don't worry about being a 'quitter'; life is about finding the path that makes you the happiest, and quitting things you hate can help you with this.