5. The Companionship/Brotherhood Bond.
Harry and Hermione/Ron or both or Harry and Remus perform a ritual that bounds them together in a companionship/brotherhood bond (telepathic link, empathic link, no secrets can be hold, seeing through each others eyes, influencing each others magic, protecting each others mind from invasions, sharing personality/appearance treats, sharing animagus forms, maybe more). Maybe it's a simple ritual but influenced by the persons magic and deepest desires (for example Harry's desire for a family).
This could also make a Harry-Hermione-Ron pairing possible (yes i know this is a lot like Harry Potter And The Power Of Three from WhiteWitchDark but her story wasn't the first one I read with this pairing and I love it)
If you think about it... it is very logical that when you spend every moment of the day with someone in your head, who can sense every thing you feel, who you are linked telepathicly and you can sense the same you begin to care about him/her/them.