I will go on diet to lose weight.
The reasons are: Firstly, I don't think lacking exercise causes most people to get fat. They get fat because of their unhealthy diet --like eating too much food with high fat content.
Secondly, I think doing exercise requires hard work and patience, and that's what I am short of.
Thirdly,I don't have enough spare time to do some sports, as I am fully occupied with my studies and work.
Finally, I prefer a healthy diet to doing exercise in order to lose weight. So I will go on diet.