It is often said that it is impossible to argue about taste. De gustibus non est disputandum. Some people like some things, other people like other things, and no amount of arguing is going to change this one indisputable state of things. This is where it is at, and thus here we are.
Nevertheless, we often find ourselves in situations where we want to convey why we like something. In matters of literal taste, the argument is simple: just present the person we want to convince with a tasting of the good stuff, and let the taste buds do their thing. Either we succeed or we do not; the outcome depends entirely on factors outside our control. Regardless of outcome, the attempt was made.
When it comes to more abstract things, such as music or writing, a similar approach is also available. Give someone a tasting of the music and writing, and see how they react. Either they get it, and your work is done, or they don't get it, and -
It is possible you at this point want to argue why that thing you like is good. Why the poem your friend is utterly indifferent to is actually amazing, why that song owns the sky and everything below it - why they should like it, too.
This situation presents something of a problem. If you really really like something, then its awesomeness is so self-evident and obvious that it is difficult to find some mean of reducing it to mere words or communicative motions. No discursive gesture would convey just how good it is, and attempts to convey it anyway often stray into unrelated territories, causing confusion or disagreement. Which, one might reasonably assume, is the opposite of what you wanted to accomplish.
A first move from here might be to simply state that you like the thing. This may or may not be useful information to the other person - it all depends on your particular relationship and suchlike. But it provides a baseline for further attempts to convey the goodness.
A second move might be to say that someone else likes the thing. Preferably, this third person is someone you both like and acknowledge as someone whose opinion matters. If they like it, then there's got to be something to it, right?
A third move might be to make a more generalized claim about mass (or niche) appeal. If it's famous, then it must be good, or it wouldn't be famous; if it's niche, then it must also be good, as it is an expression of the virtues of the niche.
As lines of argument go, these are rather flawed. But they are also very common. They are human.
Thing is. Giving reasons for why things are good or bad is hard. There are no readily available frameworks for it, and those frameworks that do exist require a non-trivial amount of effort to get in to. Most of them hide behind camouflage strategies such as the name "literary critique", and get progressively more invisible from there.
Maybe the proper thing to do is to cut our friends some slack. Give them the benefit of the doubt when their eyes get that enthusiastic gleam. -