I'm no relationship expert

You needn't be a relationship expert to all, but to your very own personal relationship it should be something to strive toward. To be constantly deepening your understanding of each other, what matters in the deepest corner of your hearts and minds that others are never privvy to is important.

In addition, empathy is a valuable tool for your relationship toolbox. Some people never show it, don't have it, or don't care enough to put themselves in the shoes of their partners. It can go a long way to understanding each other and maneuvering through a minefield of disagreements.

I think it's all been said here in various ways, but you can never hear it enough. Complacency is always a self-serving action, or non-action as it were.