Most of our emotion in language is portrayed by how its said, rather than what is said [the actual words are only around 9% contributed] so without the vocal emotions to it, it "sounds" flat and insincere. [that's why people use a lot of exclamation marks!! To show a more excited way of typing!] hope this helps!
aaaaaaa i was so busy with uni stuff i forgot to answer, sorry :(
anyways yeah dkdndkdn i know!! but still with most people (at least in my experience) texting sounds pretty sincere to me personally because of certain vocabulary and punctuation (there's no such thing as too many exclamation points) and stuff but sometimes, and i don't know what exactly causes this, even when all the stuff mentioned above is present, it just has a different vibe??? it's all about the vibe and i have no idea how to explain it still so im sorry :(