I havent read enough to get into it yet, but from what I can tell, this seems like a run-of-the-mill shounen/ slice of life. I was wondering if anyone could illiterate on why this series is deserving of a seinen tag. Thank you in advance!

Seinen and Shounen simply refer to the magazine they are published in, not the content of the work. A work aimed at younger audiences will usually get published in a shounen magazine and vice-verse, but this is not a guarantee.

This. You'll notice there are some clearly "shonen" action manga that go in josei and shoujo magazines occasionally.

Alright that clears some things up. I've been reading manga and light novels for a for a long time now and it just seemed like shounen was always a bit lighter than seinen which always seems to carry heavier content, if you know what I mean. Thanks again, especially for the fast reply. I didnt know how long it would take or if it would get a reply at all, but this fast of an answer is much appreciated and unexpected.

This is just because you generally aim to teenagers (shounen = teenager) with a lighter work, while you aim to adults (seinen = 18+ years old person) with a darker one.
In this case, anyway, Pet na Kanojo is a lot about "job and carrer decisions". It's a theme that you find more easily in seinen than shounen, so I don't find it unusual it being a seinen. Anyway, as others pointed out, anything can be a seinen if it's published on a seinen magazine. There are also cases of series that switched magazine (and so demography) while being published (for example MPD Psycho went from shounen to seinen).

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