Both dramas have simple story and not too much drama. Both female leads are cute, fluffy and funny while also having similar character roles. Also the male leads are smart and affectionate. Both shows have love triangles in the beginning but as the show progress you'll notice the female lead has interest in the male lead only.
While the overall story lines of 20th Century Boy and Girl and Because This is My First Life are fairly different, the way the stories are presented are quite similar. They shows split the screen time between couples and occasionally pull them back together for shared scenes. Also both shows have a wholesome light feeling throughout!