The reason is summarized in two words. Chemical Reactions. Our bodies are comprises of many hormones that affect the way we feel about things.
Some are triggered by hurt, others by fear and some by happiness. What determines our mood depends on how we are feeling at the moment.
Love is triggered by some hormone secretion in the body when we see someone beautiful or handsome that attracts us. The same thing happens with lust or envy.
Sometimes the emotion is jealousy or hatred. Its all about the chemical reactions triggered in our bodies due to some external influence.
It should therefor be no surprise to anyone that our moods swing at times, it just depends on the surrounding circumstances.
We are all people, for certain things that happen in our days can also be affected our partner sometimes.
It does not mean that the person does not love you anymore or that they do not want to see you, only sometimes you can not give 100% of us.
You are a couple, you are a team, when he does not give you love, do it yourself, when he can only give 10% you can give 90%
The important thing is that both complement each other