I'm really sorry in advance. I saw this thread title and immediately had to think of this
Please please excuse me. Okay now I shall read your post...
Okay sorry about that. In all seriousness now, I will do my best to describe the indescribable "love".
Love to me feels like a deep desire to care for a person regardless of any foreseen emotional obligation. Maybe it's a feeling of wanting to be with that person and see their life bettered even if it means you will have to make sacrifices or put in work of your own to make that possible. It might be kind of selfless, with the payoff being that you want to see them smile and be an even better person from the happiness you can bring them. It can be a range or combination of excitement for future happiness, fear for loss of oneself, or even nervousness like a challenge that you know you must accept in order to be with them. I think usually when people actually say "I love you" it is out of excitement, but there can also be a fear that the person will not feel mutually - especially in budding relationships.
Then there is friendly love, which can be much more lackadaisical. I know what you mean about it feeling forced to return those words sometimes. For a while I would return the words no matter what, but then realized that the lack of genuine expression in my voice was more harmful than good. Sometimes, I simply just take it as a compliment. And sometimes, I have found it best just to smile deeply with them as a form of thanks (that's why they probably said it in the first place). Sometimes a hug is in order, too

With friends, I no longer feel that I have to say it back if it doesn't feel right. I only do what feels right, and sometimes the right thing is to simply show appreciation of their kind words & presence with you in that moment. Being genuine is the most important thing for me.
I hope this helps. Sorry for the silliness at the beginning, I literally could not help myself.