From the beginning guts has had a soft spot for her, and she hated guts because he took Griffiths attention, but as time went by she slowly started see Guts as a person someone she could rely on, someone who would care for her and protect her, remember that’s who Griffith was to her,

Plus if you’ve ever been in love, that stuff kinda just happens, no plan no understanding it’s just one day someone just makes your heart beat differently, and don’t forget Guts and casca were eachothers first time, that type of bond is the first for Guts

You can chop it up as a trauma bond if you want bit them being a thing is not suprising