By default, Office365 forces you to change your password every three months. Should you want to keep Office365 from forcing you to change your password every three months, these instructions will help:
Follow steps 1 through 4 here:
This gets you to the powershell for your account. Now, you need to enter the command to disable the password change requirement.
The command is:
Get-MsolUser | Set-MsolUser -PasswordNeverExpires $true
It looks like this:
It doesn't give you any confirmation, but it works.