Imagine if a composer or song-writer sits down and writes something that is 100% an expression of their inner self. What are the odds that this would make any sense to anyone else? Very low, don't you think? Of course there is a little of this in all music and some music may have a lot of it, but the more utterly individualistic a piece is, the less it will speak to us because we don't know what it 'means'. For the philosophical background to this, I will refer you to Wittgenstein's private language argument. Let me very quickly say that music is not a 'language' and it doesn't have 'meaning' in the usual semantic sense. But it does communicate and therefore the private language argument applies. The idea of a 'private language' is incoherent because the meaning, function and significance of language is to communicate. In order for this to be the case, the meaning of the terms have to be understood. This is the function of dictionaries. Now, of course, there is the famous reply to the claim that music is a language that goes, "if music is a language, where are the words and where is the dictionary in which we can look them up"? True, music is not a language and the way it communicates is rather subtle.
But music does indeed communicate and express and it does so in its own way. There are a host of culturally specific shared conventions that composers and listeners learn. Tonal harmony offers some good examples. Within the context of a key, some notes are places of rest and others of tension or movement. Some chords are consonant and stable and others are dissonant and unstable. Composers create a fabric of expression by using these characteristics and listeners learn to hear them. It is not a language in the semantic sense, but it is expression. We 'feel' what is going on quite clearly even though it is impossible to put in words. The famous rubato that Chopin championed, where the flow of the melody is sometimes held back, then moved forward, is an example of a means of expression discovered by a composer that the listener can share. If the listener can't share it, then the piece will not be successful. Chopin however is very widely appreciated. This can only be the case if what we are dealing with is not a completely individual subjectivity but rather a shared intrasubjectivity. We feel, to some extent, as Chopin did. What worked for him, works for us.