I think that it is very interesting to try to understand the different ways that people tell each other "I love you" - each person has a different way of doing it, and, if you're not aware of that, it can seem like the other person isn't saying it.
One of the tools I have found interesting in terms of understanding this is the Five Love Languages - they have a website which you can google, since it is a commercial website hawking the books which is not allowed to be posted here).
There is a quick online assessment that you (and your partner) can do there, to compare.
The idea is that we have the languages of love that we understand and that are important to us. For us to feel loved, we need to hear it in a language that we "understand" if you like.
They have broken it down into five categories (or languages): Words of Affirmation, Quality Time, Receiving Gifts, Acts of Service, and Physical Touch
The concept goes that if you rank Words of Affirmation as your primary language, and Acts of Service as lowest, and your partner has the reverse, then them washing your car, which they regard as a sign of showing you how much they love you, comes across to you as "thanks, you washed my car, now why can't you tell me you love me?" You just don't comprehend it as a way of expressing their love, whereas that was exactly what they meant!
OK, it is over-simplified, and I can't do it justice, because there is a book on the subject that goes into it in great depth. It's a little bit mainstream self-help in style, but I think that the principles are ones that we can examine and get something from. I know I have.
The idea being that if we know our partner's primary love language then when we want to really express how much we love them, we do it in THEIR language.
"Listen, or your tongue will make you deaf." - Native American Proverb