OK, here's what I think. From having already been through it, I can say, it used to annoy me too, but parents are going to say it, regardless.. and do you know why? Because they love you and want to make sure that you always get what makes you happy - for the rest of your life. My parents used to tell me the same thing... and I would respoond the same way "how do you know"... and I proved them wrong - I met someone at sixteen, we were engaged at eighteen, and sixteen years later, we are still together. He was my first TRUE love... and after a couple of years, they quit telling me that I didn't know what love was.
I think what parents really mean by this is that as a teenager, its really hard to understand the trials and tribulations of love.. meaning.. a loving relationship. So, to clarify, I do think that teenagers and anyone for that matter, are able to feel love... a "loving, lasting relationship" is a different story altogether... this is what your parents want you to have... and I feel that parents get the two mixed up. When you say love and they say love, you bith mean two totally different things.
So, anyway, I hope this helps. To be honest with you, my husband and I are getting divorced as we speak, and its due to things that are missing in our relationship - but we always loved each other and did our best at this relationship. But we have both changed, and have decided that we dont make each other happy any more. Yes, it hurts, but all this time, until we began straying apart, we have always "loved" each other. Just that we have differences right now that we cant or dont want to work out. However, I would never agree that we never felt "real" love for each other, from the first day. That feeling was always there - it isnt any more. Sou our "lasting loving relationship" is gone... but our years of Love for each other was real. Some people may totally disagree with what I've said here, but that's how I can explain it best...