Sex is about opposition, counterpoint, like music. Imagine that you had an acoustic guitar, making a nice sound. Now imagine that you had four acoustic guitars, all making exactly the same sound. It wouldn't make much of a difference. Now imagine you had something opposed to the guitar, something with a different range, a different rythm, like a base guitar. With the opposition in harmony, it makes a much richer sound.
Think of a situation where an average girl starts dating a bad boy, and over the course of years makes him cut his hair, give up his motorcycle, clean up his beard, get nicer clothes ... she talks him out of all his bad-boy things. And then one day she wakes up and realises she's bored with the relationship, and leaves him to find somebody else. This is a really common situation. Women often take relationships on as a project. They try to change their guy, but not as a wise teacher helping them to become more of who they are - they are trying to make them more comfortable. To get rid of all the things that annoy them or push their buttons. She is changing a base guitar into an acoustic until they are both sitting around playing exactly the same song.
Music is about opposition in harmony. When you look for a partner, do not look for someone who is like you. Look for someone who turns you on, who makes your breath catch. If you find that lover, teach them, if you want, what you like, but do not try to change them. You will change the opposition that is making the sex interesting. This is also why sex with someone else is more interesting than masturbation - they're not exactly like you. Seek an experience that is like music, that has dynamic opposition. And if you find it's starting to sound the same, consider finding a different one of changing in some way. You don't need to spice up your sex life to make it exciting, you need a partner who has dynamic opposition.
In our relationships, we fight our partner every time they are different and approve every time they do what we would do. Study where you are different, and why it is uncomfortable for you. Seek out the places of opposition and instead of opposing them, study them, embrace them, make them part of your life. This way, you don't change to become just like one another and you maintain that dynamic opposition that allows for the passion that is present in amazing music and that is missing in most of your lives.