People rarely "suddenly" change, and rarely develop out of character habits. (Unless they are on drugs. blanket generalization, but I digress). But seriously, people rarely change suddenly. There are markers, there are patterns. It may feel sudden, because you carry a prconceived notion of the person you are with, and when you get tied up in your own life you may miss the subtle shifts that are totally normal in life.
Pay attention. Talk to each other, even push a little harder if you feel liek you are being evaded, either intentionally or not, even if you don't feel lik eit and would rather watch TV. Fortunately for my husband and I, most of the changes we have experienced in life have been in the same direction. If we didn't make the effort we did to communicate, we very may well have moved apart from each other, slowly but painfully, and wrapped up in our own spheres woken up one day and felt as though we had grown tired, or fallen out of love.