How come I'm not as fit as my friend even when I exercise?

Diet. If you are exercising as much as your friend but are not noticing the same results, it is probably related to the difference in the diet each of you are consuming. If you are trying to lose weight, you need to limit your caloric intake in addition to exercising. There also can be a genetic component involved, but the most likely answer is related to what and how much you are eating.
Not sure. Maybe your friend has a longer history of working out, and has established a high baseline of fitness. Maybe your exercises are different in intensity/frequency than your friend's. Or maybe your friend isn't as fit as you think!