The most similar point: Both are great romance dramas that will make you laugh and cry!

Both female leads are cheerful and work hard for their living.
Both male leads are lonely and isolated guys. He has a flaw that is cured by the female lead. Their houses look similar to me :D
They both have similar chemistry and longing moments.
Both stories happen during the winter season, so you'll see romantic snowfall dating.

Both stories delve into past lives and reincarnations. Both shows are wonderful, with high-quality cinematography. The romance of both main leads is growing throughout the episodes, and we also get a secondary couple romance plot <3
There's an overshadowing mystery that creeps in within the midpoint of the show.

Both the leads reconnect with the people relevant to his life, in his two different stages in life. Both dramas have a slice-of-life vibe to it.

The main and secondary characters are very much intertwined in both shows. The dialogues from both shows are so meaningful and wholesome, that you'll miss the appeal of the story if you're not paying attention to their conversations.

Unlike in Run On where there's a main romance plot, there's no romance between the leads in My Mister (I mean IU's and Lee Sung Kyung's characters). More external conflicts are going on in My Mister, but there is a deep emotional connection among the characters from both shows. If you like shows which are character-driven, I think you will like both of these shows.

Both of these shows have a subdued vibe and the relationships of the main and secondary characters are very much intertwined. Because this is my first life has more external conflicts going on, but both dramas portray deep emotional connections among the characters very well.

The male lead's personality in both shows is similar, he speaks less but meaningfully, speaking up his thought directly and logically. The comedic moments in both shows are shown through less dialogue and more from the characters' behavior during certain situations and I really really enjoyed this type of humor.

Both shows feature a noona romance in the secondary couple's plot. The romance in Because this is my first life is more diverse as we have three couples, and they're all showing different vibes. The romance in Run On is gentle, wholesome, and realistic.

One female lead's best friend in Because This is My First Life is also a career-driven woman who is single by choice. If you enjoyed the authoritative powerful women in Search WWW, you will like this character as well. Her story delves into a lot of the ingrained patriarchal views that Korea has and some other issues kind of like sexism and what it really means to be like a primary breadwinner especially if you're a woman. Also, her romance subplot is just like super cute.