I do this often also. Every time I start a new manga, I don't read anything else for a few days and it's difficult to pick up where I left off what I was previously reading. It's the main reason why I get behind on manga. The second reason is that they aren't completely scanlated so I have to put them on hold. There are many series that I've forgotten about because I didn't have a manga list until recently.

Character deaths have occasionally driven me away, but I usually continue reading after a 'mourning period' : after

Hide's death in Tokyo Ghoul I cried floods of tears but when I finally got myself to read Tokyo Ghoul: re I was rewarded in the end.

...Hop!

It is usually combination of bad characters and bad plot. If I don't like the manga after reading first 20-30 chapters to if I like it or not.

But I often try to give it another chance, because, like if it is highly regarded as good manga and I don't like it I'm wondering if I'm missing something. But when reading it feels like chore I just stop reading it.

I get bored easily so short manga are my bread and butter. If i find one that’s really good and completely scanlated, exceptions and all-nighters can take place (like Chainsaw Man).
I was closely following Yoru no Uta because vampires but got sick of it. Even so, it’s prob one of the longest manga I’ve read sitting somewhere around 150(?) chaps.
I seriously don’t understand how people can read chapters upon chapters of the same thing lol

-Sincerely, six angry cats in a trench coat

So, having bad characters or having a favorite character die should all be with the main problem, having a "bad or boring plot". Any problem with the story (besides bad grammar/translation), all has to do with having an issue with the plot.

Falling out of a routine/being busy with real life is pretty much the same thing in my opinion.

The first thing that will make me drop a series though is a drop in translation/grammar quality. I've seen this happen many times when a scan group drops it or the translator or proofreader changes. If I really like the series then I'll put it on hold and either wait until another group picks it up or it has an official release to buy it. If a year or two passes without either happening, then I'll drop it. But if it's not a series that's not worth the wait, then I won't bother.

The second important thing is if the plot is interesting enough to keep my attention. If the plot becomes stagnant, I'll give it a few more chapters to see where it goes. If I don't like it, then I won't finish it. ^^

I dont start something that I wont finish, I dont consider 1-2 volumes of a manga started when how many go over 20 or just start to get the plot laid out by volume 3.

what stops me from reading a manga/finishing it is always going to be inconsistent scanlations. how many I have where there are year log gaps in-between chapters, at that point it just gets lost in the pile.