Appreciation=Apology

Whenever someone gains profits, there is always someone who gets loss.   Japanese is one of the people who are very mindful of this fact. So being thankful can mean to be sorry for those who gets loss.   That's why appreciation can be expressed as 感謝 (Kansha) which literally means "Feeling of Apology."   Also there is a word "Sumimasen" which could mean both "appreciation" and "apology."  

I think appreciation and apology is just like relationship between light and shadow.   Shadow alone cannot exist without light.  Light can always produce shadow if there is an object.   We tend to pay attention to light, because it can be easily seen.  But if we can see shadow of the things, we can get close to the truth.