There are several different kinds of “I love you.” One is the kind you say to children or parents. It can be a habit, and while it is meaningful, it does not carry the same oomph as it might if you were saying it to a new relationship partner. Some families do say it and others don’t. However, if you come from a family that doesn’t say it, then you might hate it when you hear others saying it.
There’s the “I love you” between couples who have been together for many years. It, too, can be reflexive. Almost like saying “good bye.” It’s just a reminder of a feeling, and again, some people might hate it because they think it is such a special thing it shouldn’t be tossed off so easily.
The “I love you” said, with emotion, for the first time puts a lot of pressure on the other person. As others have said—if there is no reciprocation—well, that can be deadly. It is certainly very, very awkward. So people might hate it because it can not be reciprocated.
Then again, you can say “I love you” as a gift. It can be said to anyone, and it means you love the person, although not in a romantic way. It’s a way of saying the person is very important to you. Some people might hate this because they can’t do it. They might think “I love you” should be reserved for only very special circumstances. Saying it to friends somehow cheapens it.
The last “I love you” I’m going to talk about is the “I love you, man” sort. It’s a cliche now, but it conjures up images of drunkenness and the difficulty men have expressing strong feelings for each other in certain cultures. Again, people might hate it because it is cliched and it somehow makes a mockery of the sentiment. It’s a form of irony.
I guess, in the end, it’s about not cheapening the sentiment. Saying it too often or in too many contexts can seem wrong to some people. Saying it in very special circumstances when it is not reciprocated feels wrong. For some people, not saying it enough feels bad. And perhaps for all, the feeling that you have to reflexively say the same thing back can be annoying, since you really have little choice in the matter.