It has been shown that cross training is an effective (and fun) way to reach you fitness and weight loss goals
When we continually do the same thing over again our clever bodies adapts to the energy systems used and become more efficient.
This is why you may feel that you stop seeing results in terms of a weight loss goal you may have, that your best time of a 5k time trial is just not improving or that you just can't seem to increase the weight on your bar in the squat track in pump of the gym.
Depending on your goal there may be one form of training that works for you. However incorporating a mix of strength and conditioning, plyometrics, stretching and endurance/steady state based training into an exercise plan can often be more effective than one of these on their own.
So do you regularly hit the pavement, gym or prefer it in the pool? It might be time to change it up to challenge your body and fitness trying something different - even add in a few intervals to your usual lap routine, an incline to your daily jog or some time under tension to your weights.
Keep the body guessing!