Why does my Minecraft crash when i try to login?

Put the summary of the bug you're having here

What I expected to happen was...: That i could log in in Minecraft without any problems
Describe what you thought should happen here

What actually happened was...: When i tried to log in my Minecraft crashed
Describe what happened here

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...
3. ...