Everyone changes, and I can only hope it's for the better... Being in a relationship includes dealing with that change, and heck, it's not just romantic relationships, even your family and friends change, and sometimes it gets to a point where you decide to walk away.

I think it's nice to have someone you can trust and rely on as a life partner, that you know no matter what happens out there, you'll be able to return to "home". Yet at the same time, it's impossible to expect every single aspect of your life will be fulfilled by one single human (your life partner), so it's important to understand what you need from this person and what you can find elsewhere without hurting anyone.