Being in love is an emotional high for most normal people. It's a time when reality can be better than your dreams. I've heard some describe it as a time when the world changes from bleak black and white to vibrant color.
Who wants this feeling to be extinguished? No one!
Some are lucky to feel this emotional high for a long time, but for most, unfortunately, it's a relational phase that passes. Once it's over, we want it back. The main reason that feeling ends is because we don't look for solutions to our problems. Furthermore we don't process or resolve our relational anger issues. These can gradually build like water behind a dam. Slowly, the pressure can become so intense that the structure is breached. When this happens, we are in trouble. When we try to restrain this anger, we can become anxious as we fear losing control.
At the beginning of a relationship, being in love can be like a flame between the two parties involved. Sometimes the flame is a bonfire, for some it's a small spark of twigs. The amount people feel while in love can be represented by how high this initial flame rises.
But when one or both parties have unprocessed anger, it can lead to making the relationship problems bigger than they are which in turn leads to holding grudges. This is like throwing a handful of dirt on the fire. Try that every day for a few years, and see how high the heavily suffocated flame will be. A handful of dirt here, a handful there and soon there's a mound covering the fire. There are some life events, like being abused, that are like releasing a dump truck load of dirt on the fire. They will go out with a thump.
If you expect to feel in love all the time, and feel something must be wrong, terribly wrong, if it fades, then you can be setting yourself up for failure. It's not wrong to want to feel great; it's just unrealistic to have a sense of entitlement. In truth, most relationships are flawed and the key is to look for solutions to problems and being able to roll with the ups and downs as they happen.
Being in love is a relational phase that too soon goes away for most. It can be replaced by a more mature version which is typified by simply not wanting to hurt your partner.