I noticed this was an issue with me. I stopped flirting and making her feel sexy after the relationship had been official for a while. Girlfriend who I dated for over a year broke up with me because she said she didn't love me anymore. I think part of the reason was because I kinda just stopped flirting with her and making her laugh. I'm usually very good at making girls laugh and being witty in the beginning of the relationship, but then I get too comfortable or something.