The director of Transformers, Michael Bay, fired Megan Fox for calling him Hitler. That's the real reason the characters broke up in the movie. It reflects quite a bit, as the Dark of the Moon 3rd movie seemingly tries to dismantle and belittle the previous plot of Sam and Mikaela.

Sam & Jess: 18 months\n\n As she was killed in the pilot, this is likely all that will be revealed about her, so it's difficult to judge the chemistry between them. Despite this, and despite the fact that their coupling was planned by the demon Azazel, most fans agree that she is the love of Sam's life.