Minecraft shuts down!

Put the summary of the bug you're having here

What I expected to happen was...:
I expected to join the 1.7.10 server I like. But when I join it, minecraft shuts down, and the launcher opens up!

Steps to Reproduce:
1. I open up Minecraft. (1.7.10 or lower.)
2. I try to join a server.
3. I crash!