I've noticed a lot of people self promote and I think in a lot of cases it's against their best interest, I've come up with a checklist I think everyone should meet before they even consider self promotion.
How long have you been creating content?
If you've created a youtube channel yesterday you shouldn't be promoting it today, having a long standing channel or whatever would show you're dedicated to it and to creating content on a regular basis, if you've got a channel made in the past month with one video, even if someone likes the video they'd think twice about subscribing because what're the chances your other videos will be the same, or that you'll even continue making content, if you're already self promoting and you've been making content for a small amount of time my thinking would be you'll give up unless you get growth quickly, since you're so quick to resort to spam.
Have other non-biased party told you your content adds value to them?
Everyone, me included will think whatever they make is better than it is - friends will always be biased, you might have friends who're less biased than others but no friends will want to tell you something you've made is stupid and bad, strangers can be biased as well don't get me wrong, a lot of people have bad days and take it out on strangers but it's fairly easy to spot, has a stranger told you not only that your creation is good but what value it's given them, made me laugh, helped me with x things like that
Do you add value in this area?
This should be fairly self explanatory but extends beyond just a viewer/subscriber/whatever basis, do you add value to the larger thing you're contributing to, if you're a vegan fitness youtuber do you add value to that genre and part of youtube? You should display not just your value to the consumer but to the group you belong to, anything less makes you seem selfish and people will be reluctant to support you. This doesn't mean be nice to everyone but care about the genre as a whole, burn bridges by calling out shitty content creators, help under appreciated content creators, you can't expect from others what you're unwilling to give to others.
Do you do anything differently?
What separates you from the others that create content that falls into the same category of you? doing something differently helps you stand out and gives you a reason to picked over others.
What's your current rate of growth?
If you're growing at a consistent, steady pace then think twice about self promotion, the gains are fairly small and even if you have the perfect content some will still dislike you just because you self promote.