Just curious on what makes us lose interest and quit our diets or healthy lifestyles?

I think part of it is that we're all waaaaaaaay too hard on ourselves. We forget that the simple pleasures in life should not make us feel guilty or inadequate, they ought to be sweet little pick me ups that add variety to our lives. We often are so strict when it comes to diet that we can cut out all the things that makes food so gosh darn enjoyable!
I believe that if we all adopt a healthy outlook, then our changes will be much more realistic and we will see more long term goals. When you adopt a healthy lifestyle it shouldn't be for today, it should be for tomorrow, i.e. The Future.
The excercise thing gets boring, but, if you understand that every activity has a value you'll be laughing! Walking the stairs and even shopping, for goodness sake, burns so many calories (as long as you stay away from the crap the malls sell in the food courts!)
So I say, enjoy your no-fat non-whip vanilla bean latte! Enroll in a wacky, yet fun fitness activity like Stripacize! And, $#%! the nay-sayers and get down to it! Who on Earth told you you can't have that piece of pizza!? Well, I've just worked out, I've eaten carrot sticks for a week and I deserve it!
I deserve that one slice, if only for my sanity!