Why is it wrong to be fat?
It is not "wrong" to be fat, but it is generally not viewed as a "good" thing, either. Many cultures and media sources do not view being overweight as being attractive. Some do, but a majority prefer a healthy or slim body type. Also, being overweight can cause amny health risks or problems.
OK, one: You spelled impress wrong. Two: It doesn't matter if your fat. That right girl for you will love you for who you are, and she won't care if your fat. I know a lot of girls don't like fat boys, but then there are those nice, reasonable ones who actually care about what's on the inside, not just about what you look like.
Ultimately, if you gain weight, it means you take in more calories than your burn off. It could be that what you consider exercise is not be rigorous enough to burn fat (sweating doesn't mean you are burning fat) or that your workouts are too short to burn fat (20 minute minimum on the treadmill, for example), or maybe you eat/drink too much of the wrong things at the wrong times...etc.