To know someone who you think could understand you entirely, with a bit of time and effort as is usual, and yet who cares not to... this may be worse than never meeting or knowing such a person, which is a great sobering sadness itself. At least then there is a certain comfort in thinking that it is because you have never crossed paths. But if you have, and they do not wish to care, how can you not take that a bit personally?
What's worse is to not be let in to understand them; being understood and appreciated means little if it is not done in return. Still, what can we do but persevere and try to maintain some optimism, hoping to be useful to people when it's needed and not let self-centred introspection get in the way. A worthy aim, or a forlorn hope.