Why do things change?

Why do men change after a timeframe of being together?
I’ve been with my partner for a year and 3months. We’re with eachother daily, is that why? We hardly kiss, unless I initiate it first unless he’s leaving for work, he will initiate. We don’t really hug unless I ask or do it, don’t hold hands, the physical affection isn’t what it was in the beginning, I know things change over time and die down. But why? I still want to do all that stuff/more often then we do, But it seems he’s fallen back? I noticed this in all my relationships, so I feel like it’s normal but why? We’re currently looking for a place together, my daughter and him have a good relationship. She’s even slipped and called him dad twice. Which has never happened in any other relationship before. Our relationship is good but I’m a worry wart with everything and over sensitive and constantly think I did something and that’s why it has died down but he tells me things just change over time which I also understand. Help? I want to ease my mind, Thankyou.
Why do things change?
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