Why am I so ungrateful? Why can't I just accept the kindness, and not feel the longing for more?

I think the depth of the longing is a mirror reflection of the depth of the need, and the need isn't fulfilled until it's fulfilled. That doesn't make you ungrateful, it makes you a feeling human being. My guess is that why you don't feel this longing with your friends is because they don't reflect the core need. They don't activate it, so you don't need them to fulfill it.

You're in transition, in the middle of it all, so you can't envision the need ever being fulfilled, but it can be. You just have to be patient and stay open to it. I think the fear that the love will overwhelm you is a red herring, or a defense. The fear is real, but it doesn't reflect the reality.