Whats wrong with doing only cardio?

To start with, cardio is good way to lose weight. Issue is that as you get fitter, your body can't push itself to burn more calories to keep the cardio up unless you make it stronger. I'm doing more cycling, so am doing strength training alongside to make my body stronger to cycle faster and harder, and recover faster. Leg presses etc. My training buddy does no strength training and joined me on a cycle ride on Sunday morning and was way behind. I could also walk on Monday morning and she was in screaming agony! Depends what you want out of your exercise regime. If you want to walk a long way without getting out of breath and keep your weight under control, doing only cardio will probably be fine. If you want to take on more sporting challenges, then you need to do both.