Being in love in general makes you happy.
Having someone to hold makes you happy.
Knowing someone cares about you makes you happy.
You have something to do besides make yourself more miserable.
You're not as focused on yourself because you have someone else to think about-- it's a good thing.
Knowing someone likes you makes you feel more likable.
Seeing them knowing you like them makes them feel more likable, which makes you happy.
You have someone you can always talk to about anything.
Sex releases endorphins, which make you happy.
You're more motivated to do things because you actually have to consider what they want to do, and being active makes you happy.
You don't end up sitting at home and driving yourself crazy all the time.
If you do something, you aren't just doing it, you're doing it with someone else.
You can share the housework, split the cost of things, etc... which makes life in general easier.
So yeah, a good relationship definitely can change your life in very positive ways... it's not about being co-dependent or hoping someone will solve all your problems so much as introducing an element into your life that demands you start acting, thinking and feeling differently.