I read this world famous book maybe around 2012. Gary Chapman proposes that there are five main categories of how people show love to each other:
1. Words of Affirmation
2. Quality Time
3. Receiving Gifts
4. Acts of Service
5. Physical Touch
Due to the fact that people show love in different ways, we may not always feel loved by the other person who has a primary love language different to you. The names of the love languages are quite self explanatory but the book gives examples and scenarios of how these love languages manifest itself. It's quite useful to know the primary love language of your friends and romantic partner so that you can adjust your behaviour to fit their needs.
I also quite like Gary Chapman's idea of a love tank. When you feel like you can't love this person you used to love because of what he or she did or say, Gary Chapman proposes a six month experiment. He explains that when our love tank is low, we have no love feelings but only experience emptiness and pain. Since love is such a deep emotional need, the lack of it is perhaps our deepest emotional pain. If we could learn to speak the other person's primary love language, that emotional need could be met and positive feelings could grow again. For six months, speak your significant other's primary love language consistently, somewhere on the line, that person's emotional need for love would begin to be met and as their love tank gets filled, they will reciprocate their love to you. That hypothesis is built upon the idea that the emotional need for love is our deepest emotional need and when that need is being met, we tend to respond positively to the person who is meeting it.