Environmental pollution already has been mentioned.
There are positive aspects of tourism though. It depends on which kind of holiday you want to have.
Spending your time only in the most crowded parts, the typical tourist areas often means missing the cultural characteristics.
You hardly learn anything about the country and its people.
In other words, you can learn a lot if you are really interested in a country.
However, I can understand those who prefer going on holiday just to relax a few days.