Disclaimer: OMG, so totally not mine. :P
Reasons
Maria Ross doesn't think she's the type of woman who'll ever have a home, a family. She is a soldier; she has her career and her comrades. Usually that's enough.
That is why she is so surprised how deeply the Fullmetal Alchemist and his brother affect her, how sharply the ache when she looks at them cuts into her heart, how fiercely she feels the protectiveness they rouse in her. They are children, she thinks, and children shouldn't have to face the things they have, fight the battles they do.
That's why Maria fights, so that they won't have to.