Let me start by saying, life and marriage is all about derailment. You must be thinking WTH or how scary is this right? So give me a chance to elaborate. When has your life ever stayed on track or on the same track that you have been on since the beginning of your life? Unless you're a robot or a cyborg, which would be very cool, I'm sure there have been times when you were thrown "off track" in life. We rarely stay on the same track in life, not because life sucks or is unfair, but because we are constantly changing and evolving as people. You need to change tracks in life when things are not going well, when you want things to get better, or when you find that the set of tracks you're on has taken you as far as you needed.
Ok, so now I'll get to the point about how this relates to marriage. It's impossible not to change and impossible to change others. Sure we can influence others, but it's their choices/behaviors that create the changes we see. Before looking at how your partner has changed, I encourage you to look at how you have changed since getting married. Has your patience become thinner, have your criticisms increased, or have your encouragement/support decreased?
Imagine your marriage or relationship as two trains connected on one track. One day you might wake up realizing you're on a different track, and without notice your partner merged onto a new track taking you with him/her. The merge was so subtle you could not detect it, but now your relationship is going in a new, perhaps unknown, direction. This is how subtle real life change is. It's undeniable that small subtle changes in each partner, can create a new direction in a relationship. What I want to impart to you, is that these "merges" will happen, with or without you being in control. So I encourage you to be aware of your choices and behaviors, which is the only way you can effect the direction of your relationship, and choo-choo-choose the right track for you.