The following article is a pretty gentle one, on the topic:

Exploring the Reasons Behind the Popular Trend of Liking One’s Posts on Social Media

Narcissism
An overwhelming sense of self-love and self-importance characterizes narcissism as a personality feature. People with narcissistic traits frequently put their needs and goals first and have no empathy for others. (...)

To Climb Feed
Climbing the feedback ladder is a psychological phenomenon where people seek positive feedback from their peers and is often the cause of this behavior. (...)

Due to insecurity, people like their own posts
It’s becoming more and more typical for people to publish anything on social media today and then like their posts immediately. People who engage in this conduct frequently exhibit insecurity because they may need to affirm and legitimize their own posts. Some people might appreciate their posts because they want other people’s attention or approval. (...)