“In the summer I seem to sweat excessively, more than I should be. Why?”

You could just have a naturally higher metabolic rate so you produce more heat and sweating is how your body avoids overheating. Or, if you are overweight, the insulating effect of the fat layer may make it difficult for your body to lose heat without sweating a lot.
If this is something that has changed quite rapidly (over a year or so) it would be worthwhile having a blood test for your thyroid.

Some people sweat more than others. Check your thyroid functions and blood sugars.