For me anyway.

I find the whole idea and process of having a friend/pet/creature come into your life and it is small, scared, weak, injured, what have you,

and you nurse it back to health, you take good care of it, protecting it, snuggling with it, giving it all your love. You have adorable little precious adventures.

Its small enough and helpless and you are its best friend and you two have so much love for each other.

And your friend/pet/creature grows, and they grow and grow. Its just a natural growing up process, not like size changing.

And eventually they are bigger than you, huge in fact, gigantic. They have grown up so much bigger and stronger and fiercer than you could have imagined.

Now their size can be daunting. Big teeth, sharp claws, a much bigger mouth to feed...
But even at such a size, they will start to be the one who is protecting and taking care of you. You belong to them now, and they will make sure no one harms you or that you are in any danger except perhaps from themselves. And that can be in a predatory or playful way.

Point is, now you are the little small and helpless one. Sure you can take care of yourself, but now this big creature is in your life and they could be thinking about you in two ways:

1. That you are still their protector and they love you and love being taken care of by you.
2. They realize they are bigger than you and can get away with anything they really wanted, and that can get adventurous.

Now they can also realize that you're no longer needing to protect them or take care of them and that they will get along just fine on their own, but they want to stay with you anyway because you guys love each other in whatever kind of way, and it will still want you there to take care of it even if it seems silly.

The roles will of course reverse in time and you will end up being their little pet/friend/creature and you'll have this incredible, powerful, amazing companion that will stick by your side and love you, play with you, protect you, and take care about you.

Then depending on how you roll with your fantasies you guys will undoubtedly explore them all.
Vore for the win!

The birthday present I got from my Rex made me realize how much I actually love this scenario, and I will utilize it in my future endeavours!