The sun is so critical to the water cycle because if their was no sun there would be no evaporation and the whole Water Cycle would collapse. It also is important because maybe there would be no more water in our life.
The sun is middle aged. Our star is in the middle of its life cycle, and if you use the link below, you can easily read a short post on what you can expect as the sun ages. It's worth the short jump to Wikipedia to do that. Really.