Both dramas star Jiro Wang as an arrogant popular singer that has never experienced love, only to soften as he falls for the female lead. In both dramas the female lead is shy and rather unworldly, and the hero is protective of her, although at the beginning he doesn't understand why he cares so much. Both dramas also show the two leads living in the same house, so both shows feature a lot of sweet cohabitation scenes.

However, Fabulous Boys has more characters, so it develops the side stories perhaps a bit more than ToGetHer. I also felt that the hero in ToGetHer went through more of a change than the one in Fabulous Boys, because apart from falling in love he also loses his popularity, whereas the character in Fabulous Boys never quite reached that level of vulnerability (he was more arrogant and stayed it longer).

Both are romantic, funny and light, with a little bit of drama towards the end (but not too much). Also the actor that plays the manager Jie Ge / Tony Ge is the same in both dramas :p

Both dramas are based on chinese internet novels written by the same author, Gu Man.

The stories don't have much in common but I have read many comments (and I agree) finding the two male leads similar. It's true that they seem to have a cool and collected façade, but both have a very strange, irrational part to them that is revealed as they fall in love with the female lead. One of the most striking illustrations of this is their "demented" laugh. Also, both actors (So Ji Sub and Jang Hyuk) are more "mature" actors (37 and 38) that play mainly in action type dramas, and both Master's Sun and Fated To Love You are their first rom-coms in years.

Both Reply 1997 and Hot Young Bloods follow the stories of teenagers in the recent past - Reply 1997 takes place in the 90s, and Hot Young Bloods in the 80s. They both pay careful attention to the aesthetic details of the era. But beyond the fact that they’re both “retro”, they also both portray characters that are normal, relatable teenagers. None of the characters are outrageously rich, celebrities, damsels in distress or alpha heroes… Both female leads are a little on the “strong” side, they’re more assertive than the male leads, that are slightly timid. The male leads, despite being awkward and flawed (and occasionally ridiculous) teenagers, are still attractive. And both this movie and this drama have a nice mix of romance, comedy, and slight notes of sadness, that sums up into good coming-of-age stories!