at least Kenzie didn't totally change Chad.

It was a little weird that she was in high school when they got together and got engaged. And the fact that they have been engaged for 4 years is really weird too. The whole point of getting engaged is to say we are getting married and starting to plan the wedding. It's an in between stage... and in general it's not a good idea to spend a long time being engaged. So there's clearly a reason they have been engaged for 4 years (dated for only a few months though... also weird).
I really don't understand the point of proposing when you don't plan on getting married. I think Chad has the "Lucas disease"... he's in love with the idea of love and will turn to whoever will provide him with that love. It's like he thinks the only way to show someone you love them is to propose marriage.