Orphaned at an early age, she was brought up with Eomer and Theodred, with no girls or female role models in her life.  Any other women would be of a much lower status, she was the granddaughter of a King.  This was a society that valued male roles (especially warriors) very highly, so she must have felt very out of place as she reached an age in which her only companions were out fighting a great deal of the time.  Then, as Theoden sank deeper into his depressed state, she became his caretaker.  Quite a lot of pressure to be under.  Had Theoden been fully himself, he would have arranged a marriage for her, but he seems incapable of that, leaving her potentially for Wormtongue (not an attractive prospect).  So, when Aragorn comes on the scene, she would naturally have pinned all her hopes on him.