Love Compatibility is complicated. You don't know what happened. In the beginning, he was funny, charming, generous, and caring. But now, he's lazy, jealous, boring, or predictable. Sure, he was great when he was showing you his Sun sign in the beginning, but now that he has what he wants, he tries to get what he needs.
In astrology, the Sun represents the ego. It's what a person hopes to be in the world. Likewise in astrology, the Moon shows what makes a person secure, what they fear, and most importantly, what's important to their survival. Thus, Sun signs show what someone wants, but the Moon shows what they need. The Sun is active. It's what we are and do. The Moon is reactive. It's how we feel and react without stopping to think. We need what can be taken from us, or what we think or know isn't readily available.
For example, you may want a million dollars to fall into your lap right now, tax-free. But you may not actually need it unless you have a million dollars in debts or expenses. The Sun wants a million dollars. The Moon needs the million dollars. The Sun doesn't have a nagging reason to have the money, even if it would like it. The Moon has a deficit that the million dollars would fix.
We may see demonstrations of a person's Sun sign when we meet them. A Sagittarius may seem happy go lucky, and a Pisces may seem dreamy. An Aries may seem like a go-getter, and a Virgo may seem cautious. But Sagittarius doesn't necessarily need optimism to survive, and a Pisces doesn't actually need to live in their heads. We may not see this right away, but their Moon sign shows what a person actually needs, and how they get it is shown by their Sun sign.
An Aries with a Virgo Moon may seem like a go-getter full of confidence, but what it actually needs is seen in Virgo Moon. It needs to be competent. It may fulfill the need to be competent through Aries-type things, like competing with others. Likewise, a Pisces Sun with Gemini Moon may be dreamy and absent-minded, but it's actually paying attention to everything because Gemini Moon needs to know everything that's going on in order to feel secure enough to daydream.
A couple has to have harmonious needs if they want to get along and share a life together. Moon sign incompatibility is one of the main reasons relationships don't work out, even if the Sun signs are compatible. On the other hand, Moon sign compatibility can actually make a relationship work when the Sun signs aren't compatible.