Okay you guys - so here is my entire theory on Sam's past.

In the 14th ep "Primed" there's a case Sam seems to take very personally. Sam gets very angry and acusses the stepdad of their missing girl that he "diddled her" as its said. She is not right and jumps to conclusion which is very unlike her.

So what I am thinking is this; Sam obviously has trust and commitment issues - those most likely came from her childhood and could be from her mothers different boyfriends. She couldn't allow herself to get too attached to any of them because she knew they'd leave again and she saw how much her mom hurted every time they did.
I have a suspicion that she might have been abused - could be sexually but its hard to tell.
I believe the abuse and the unstableness at home caused her to elope with the first the best loser she fell in love with, though she learned the hard way that it wasn't the way to live life - he could have abused her too, caused her to miscarriage or give up her child for adoption. I believe that's why she came to New York and became a cop - in order to learn how to defend herself and help others out of the misery she once was in.
I think Jack knows this - or bits of it and that is why he still seems to feel deeply attached to her and seems to feel obliged to let her know everything that is going on in his life. In other words; the reason he told Sam about Anne and her pregnancy as the first one even though Anne hadn't wished for anyone to know was because he felt he owed it to Samantha to let her know as the first at least seen as she did tell him quite a bit of her story. This makes him a part of the problem for her.
The guy does have a degree in psychology and he knows that more let-downs and breaks of trust in her life could break her down completely and he doesn't want that.
Here is another thing I have to mention; Sam seems to have father issues too. Jack symbolizes a person in whom she put her trust and blurted her heart out to. She obviously never had a real father figure in her life and Jack became a replacement. That is why it hurt her so much when he a) broke it off  and b) left to go with his wife. She saw her only stable figure of trust and comfort leave her for good and that crushed her.
That was when she turned to Martin - in the beginning just for comfort and a good f*** but that is not his thing. Martin wants more than just friends with benefits and it grew on her.
He showed her new aspects of love and it made her grow a little each day. So when he broke it off with her she was very hurt and realized just how much she'd grown and she felt as thoughs she owed him something in return and because of that and because she feels something for him she vowed in Showdown that she would be there for him. That was also what made her confront him with his problems in "The Road Home".

Now lets have a look at season 5 - Sam has grown a lot but she still have issues in her past that she's got to face.
I see a more sensitive Sam in season 5 and I believe her past is catching up on her. It is coming back to haunt her because you can only hold it off for so long.
She has realized that clinging to Jack won't help her or move her forward - when he left he broke her trust. And I know he returned quickly but it was the fact that he was willing to leave that crushed her hope and faith in him. The days where she could trust him are over. He might still be attached to her and they will be friends but nothing more than that.

Sam is the helping type as well - a good girl. A child takes the place it feels that is missing. In other words - Sam is taking care of others and defending them because she missed that when she was a child.

I believe that when Sam returns to Kenosha it will be because she realizes that she has ghost in her past that needs to be dealt with and that will hurt a lot.
I believe that Martin will be there for her - he is the type of person to do so and he still cares a lot for her. That is who he is - he takes care of people and Sam is one of those people.
I know I'm a Smarty fan so of course I hope they'll get a second chance and that is what I see is most likely to happen but the theory on her past and her relation to Jack has got nothing to do with the fact that I am a Sam&Martin shipper.

Time will tell if I'm right or not but this is what I see.

*hugs* Helene