Put your head on my shoulder is mainly a light hearted romcom, but if I'm watching a show where the romance is the main sell, I am going to be quite critical of how well the leads suit each other.

From that perspective, I don't think Gu Weiyi and Situ Mo make a good couple. GWY is a tinkerer and nerdy type at heart, he loves taking things apart and putting them back together to see how they work. Situ Mo is a pretty average nice gal. There's nothing wrong with that, but her personality doesn't not complement his.

For example, she's dismissive at best of his tinkerer nature, such as the time he turns off the lights and shows her his hand crank powered lighting device. Rather than being supportive, she points out that even her phone screen is brighter than it, putting him down. This is generally the case whenever he displays his nerdy side as well, she responds with mild annoyance (think roll your eyes type of response).

The bottom line is that rather than bring out the best in him, her personality pretty much eliminates what makes him special and by the end of the show she's turned him into a bland generic nice guy lead. He even states in voice over in the last episode: "when I'm with her I feel like my IQ goes down to 87." (this is not meant to be negative in the context his voice over, but it still illustrates the type of effect she has on him)

The second male lead, while annoying at the beginning of the show, actually goes through more development than the male lead in the show and by the end of the show, I appreciated the second couple more than the main couple.