"That relationship was a waste of time." Really?

Anyone else heard someone say (or said themselves) that a relationship they were in or friendship that just ended was a waste of their time? I know I sure as hell have and it aggravates me! Don't get me wrong, sometimes they end badly (...ALOT of times they end badly....well mine have >.< ) but before that ship gets wrecked it would be a lie to say that you didn't have ANY good moments with that person. Furthermore, there had to be a at least one lesson you took from that interaction (no, not trusting people isn't what I mean). We fail to realise that we are the sum of our interactions. 

Think about who you are right now. Would you be the same person if you never met any of your ex's (friends or lovers)? If you can say yes to that question then you're lying to yourself and I feel a bit of pity for you because you may end up repeating a few mistakes for a bit longer. Maybe I'm wrong and you're that one Super person who hasn't needed people -- but I feel if you truly look back you will realise how you've been changed by your interactions with an ex.
From my experience I've lost a few friendships and when I look back I see where I messed up (of course you can look at what they did and dwell on that but you can't change how anyone interacts with the world except yourself) -- going forward I can make sure I don't repeat the same mistakes (it isn's a perfect transition but changing is a part of growth and that is much better than the insanity of repeating the same habits and expecting a different result). The same applies for ex lovers, its so easy to say how horrible the were or how they did that one thing that irked the hell out of you...or you could recognise your own failings to see how to make better choices in the future.
Now don't get me wrong. I'm not saying you need to blame yourself since you are the sole problem, nor am I saying that the negative aspects are the only things that have made you who you are. People who are no longer in our lives have shown us how to open up to others in new ways or see the world through different eyes. Their experiences (and our experiences with them) have allowed us to learn more about who we are, likes and dislikes -- hell they've given us good memories too (memories that sting when we get angry at ourselves for allowing them in or thinking about how they left). Our past is simply a part of our story and there is no reason to be angry because there is no way for you to change whats been done. But as we go forward we have our experiences within our arsenal so we are better equipped for our future interactions. 
A relationship has never been a waste of time and it was there for you when you needed it (even if you didn’t realise it at the time). Love the memories left by those who aren’t in your life any longer and make sure you that you take the lessons they gave you persist as you move forward (as long as you need them).