When you are trying to come up with cute things to say to your girl, to make her smile or laugh, or simply to make her feel loved or appreciated, you had better hope that you know her very, very well. Knowing your girl well if you’ve been with her for a while seems obvious, but it is often taken for granted, to the detriment of both people involved. Knowing her well can mean the difference between being successful or unsuccessful in finding cute things to say to your girl, and therefore between making her happy when you talk to her or making her indifferent. If you extend the implications of that, it can mean the difference between continuing to know her very well or not knowing her at all.
Anyone around your girlfriend can be cute, charming, funny, etc. It could be a male or a female, a coworker, classmate, professor, employer, a random person in line for coffee, etc. She is, on a daily basis, surrounded by many different influences and members of her environment that can bring out the positive reactions you want your own interactions with her to bring. You, however, are not simply a member of her environment. You are supposed to be the most important member, the most important person in her life, and the one most likely to charm her or make her laugh. This can be a blessing or a burden, or a bit of both.
Either way, it is your job to stand out from the rest of the people around her, and make her view you as special and irreplaceable, through how you are around her and what you say to her. Finding a few perfect cute things to say to your girl is certainly a big part of achieving this, and knowing her extremely well is, in turn, a big part of finding the perfect cute things to say to your girl. When you are in a long term relationship with a girl, she will, ideally, show you parts of herself and her life that she rarely (if ever) shares with anyone else.
Beyond this, you are (presumably) around her so often that you easily pick up on her mannerisms, her quirks, her tendencies, etc. In short, you often know your girlfriend better than anyone else in the world does. Because of this, you should utilize everything you know about her every time you try and make a cute comment to her, compliment her, or talk to her in general. If you want to make her laugh, if you’re looking for cute things to say to your girl, then you should use everything you know about her, playfully make fun of her, tell her how cute her laugh is, or the way she yawns, etc, and tell her how much you love her little quirks that make her unique. These comments, and others of the same nature, will work wonderfully and make her view you as a special and amazing boyfriend.