Smh the one complaining about this seems does not read the story carefully, it all started with insecurities between them about their relationship after having sex, mako fears that shinici only love her for her body (sex) and shinici fears that mako doesn't happy or enjoy with relationship cause she does not cum when they are having sex. When you are insecure misunderstanding still can cuase a problem (when you already experienced how relationship is you must have known this).
In the end shinichi wants to show mako that he loves her by showing or doing what she likes and want (movie and doll she likes in their date) plus shinici gives a REAL ring to show how serious he is with her. And mako finally believe in shinichi that he really loves her by recalling what he already did for her from past till present.
Note : and this romance is mix with comedy so some parts maybe funny and dumb but that just how comedy in this manga (they are yelling at each other since the beginning, if you think some other things dumb think first that they yelling each other almost every chapter manga that obviously no way in real life that happens)