Existentialism...NOT Explained

Existentialism. Where to begin? Even after our discussion in class on Monday, it's hard for me to understand the logic behind this stance. Don't get me wrong, I don't think that selfishness is always bad.  It's just that I think existentialism is a bit extreme for my taste. I couldn't find many relevant videos on existentialism to supplement this post (the ones I did find made me question whether they were school-appropriate), so instead I searched quotes on existentialism.  The following quote really stood out to me:

"There is something infantile in the presumption that somebody else has a responsibility to give your life meaning and point... The truly adult view, by contrast, is that our life is as meaningful, as full and as wonderful as we choose to make it." - Richard Dawkins, The God Delusion 

This quote seems outspoken to me. I was searching videos and quotes not only to supplement this blog post but also to try to understand why existentialists believe what they do. However, this quote just confused me even more. In my opinion, there is nothing wrong with feeling a certain way due to someone else. I don't believe that one should let others control them, but it's okay to feel happy because of someone else's actions.  For example, if someone throws you a surprise party for your birthday, it's natural to feel happy. So I don't know why Dawkins thinks that this would be an example of someone else having "a responsibility to give your life meaning and point."  

Moving on to the second part of this quote: 

"The truly adult view, by contrast, is that our life is as meaningful, as full and as wonderful as we choose to make it."

As much as I dislike Dawkins' quote, I actually agree with this part. I believe that unless you actively make a difference in your life, there won't be any progress. What I don't appreciate is his statement that this a "truly adult view." I am only seventeen years old and I most certainly lack the maturity of an adult, but I am aware that I choose to make my life a certain way.  I know that I choose to be a positive or negative person. I know that I can be a better person or a worse person. So for Dawkins to say that this is only an adult view seems almost hurtful and harsh because it seems like he's belittling teenagers.
I may be taking this quote completely out of context. Perhaps if I knew why he made that statement or what situation he was in, my perspective would change. But as a student who strongly disagrees with this philosophy  this is my opinion.