Rain comes with greatly reduced visibility, turbulence, and low cloud ceilings. You need VMC to fly, that means, cloud ceiling of at least 1000ft AGL, and visibility at least 5000m. Maybe some other minima as well, such as moderate to severe turbulence. Also, the rain can damage the prop.
Jayke, as a pilot with my certificate, I look hard and long at the aviation weather forecasts, before I fly, and look for any excuse not to fly. Remember, we fly for fun, and such conditions wouldn't be that much fun to fly in. As SSCBD said, talk to your instructor about it.
I'd rather be on the ground wishing I was in the air, rather than in the air wishing I was on the ground.
Trust your instructor in these matters, you will come to respect the wisdom in his decisions.