According to Dr. Barton Goldsmith of "Psychology Today," nice things to say are ones that make the person feel loved or appreciated. This could include such statements as "I love you," "I'm glad we're together," and "I appreciate you."
If your significant other feels that you are not saying enough kind words, remember that sometimes, it is better to risk sounding a little sappy rather than miss out on important intimate moments. Be natural and expressive. If your boyfriend or girlfriend looks nice, say what you are thinking. If you are having a special moment, verbalize how much you appreciate it.