I think there's a difference between technique, and the spark.
If you have really great chemistry with someone, a really great spark, you're going to feel that even if his/her technique is not what you prefer.
On the other side of the coin, someone can be an expert kisser, but if the connection isn't there, all the technique in the world can't make up for it.
I think it's a sign of love and trust to be able to help the one you love learn what pleases you, just as you try to learn what pleases him/her. Sometimes you don't even have to discuss it. You can learn a lot just by being observant of your partner's body language. I think if you have a real connection with someone and you're really into them, you're going to notice if he/she has any "hot spots" or things that he/she really enjoys, and I think that if that person feels a real connection with you, he/she will notice the same.
But sometimes, you do have to give hints. And that's okay. A person who really loves you will want to know what pleases you.
All that being said, if you don't feel the spark, the connection, the chemistry...there's no point.
ETA: As for losing the spark, all I can think is that maybe we develop chemistry with someone based on the image of them that we fall for, and if something happens to distort or change that image, if we learn something about him/her we didn't know previously, that spark might go away.
We see the world as "we" are, not as "it" is; because it is the "I" behind the "eye" that does the seeing.