I thought of a head canon last night but figured someone already did it. BUT anyways 

Bertholdt and Annie use to date, they had a really strong relationship and Bertholdt really loved her, but when things started getting messy in the walls, Annie broke it off and Bertholdt could never get over it. Which is why he cares so much about the mission now. Because he wants it to be over and back in his home town with her so that they can be together once again.