To the core, human beings are selfish. Yes, people can be well-behaved and treat each other nicely. People do put other people before themself in their lives, and some are better at this than others. But even those people whom you put in the highest regard will eventually let you down. No one is perfect.
This can frustrate and disappoint me immensely, but that very fact shows that I am no better than any of them. When I am bothered by people who do not live up to my expectations, I am putting myself first instead of being a friend for them.
Other people will from time to time disappoint. I will most certainly disappoint. There is only One who will not. What a friend we have in Jesus!