The scientific evidence that drinking lots of water is good for you is very shaky. There is a tendency to believe that if a certain amount of something is good for you, then lots more is better, which is rarely the case.
The theory is that you should take in 1 ml of fluid for every calorie consumed - for the average person this would be 2.5 litres of fluid.
But this can come from the water content of food, as well as drinking water - and other drinks. A lot of fruit and vegetables have a high water content.
If you are happy drinking so much water, on top of other drinks, then fine. But if you are forcing it down, or it is interfering with your daily life, work or sleep patterns, then maybe you are drinking too much.
I have got into trouble for saying this here before, so feel free to ignore me if you like. But you might like to read the following:
Drinking Loads Of Water Does Not Seem To Improve Health
Evidence lacking on health benefits of drinking lots of water
Drinking eight glasses of water a day is healthy, right? Wrong. - Health News, Health & Families - The Independent