When ever i try and log onto minecraft it tells me error, i press play and then it doesnt go to the title screen it says error and that it crashed

Put the summary of the bug you're having here
Every time i load up Minecraft it doesnt even load up, it comes up with error and that i crashed, i am currently on 1.7.2.

What I expected to happen was...: It to load up to the title screen but when i pressed play it didnt do that

Describe what you thought should happen here
Load up to the title screen where i could select my world ect.

What actually happened was...:
Describe what happened here

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ... I pressed play
3. ... then it came up with the minecraft 1.7.2 menu and said it crashed