There are stories where characters are isolated or are in competition with themselves. These kinds of stories are hard to write and can easily come across as self-important and self-indulgent. Everything’s about him, nobody else counts, he has to do it all by himself.
That’s usually not the intention, but it’s hard not to come over like that if every sentence starts with the same subject. However, once you bring in a rival to your main character, things not only become more dynamic, they also help the reader see the main character more clearly.