When I try to login with my Mojang account it says "Login Failed". Why does it do that?
Did you buy a legitimate copy or are you using a cracked version? This is a known issue with cracked copies of Minecraft.
If this is not your problem then there really aren't many other reasons.
- The login servers could be down.
- You're typing something wrong, recover your password just in case.
If these aren't the problem, then email Mojang. Also, check this site for info on if the servers are up: http://xpaw.ru/mcstatus/.