2. A Father’s love
This is obviously one of the more obvious reasons, but I think it should definitely get a mention. Thankfully Rogue One: A Star Wars Story finally fixed a near 4-decade plot hole in Episode IV by finally explaining just why the Empire didn’t fix the exhaust port issue. Galen said himself in his transmission to Jyn that he needed to work on the Death Star because by doing so he was in the perfect position to sabotage it.
His main goal was obviously to give the Rebel Alliance a fighting chance, but it was his love for his daughter that made Galen sacrifice his freedom. Galen wanted Jyn to grow up in a galaxy free of oppression and the best chance for that dream to become a reality was to sabotage the Empire’s greatest weapon. He knew she would grow up to resent him so Galen didn’t only sacrifice his freedom but, his daughters love.