Why oh why do we tend to always wish for more?

Bismillahirrahmanirrahim.

Salam alaik and greetings to dear readers,

I'm going to discuss on humans greed and lust (nafsu). So, we all aware that we should always give thanks to Him for everything that He has given unto us and yet, we still continue to always wish for more and always comparing ourselves to those who are better off or more fortunate than us. Why? Why are we not easily contented? I have to admit that I too wish for more at times although whenever I feel unlucky, I will always pick myself up back by reflecting on those who are way less fortunate than me, who lives a tremendously difficult life compared to mine. With this, I always get to be on my feet again with the feeling of overwhelming gratefulness with the life He has patched specially for me. However, I do wish for more on a few occasions; especially in times where I face a hard time understanding my learning materials. This is when I start to wish, "If only I could become smarter. Why didn't God give me a better brain? Can't You just help me understand these fast?" and it goes on incessantly, it never ends. And after I speak of these words, what follows after is this train of thoughts that plays in my mind:

--Am I not being grateful? My complains makes it look as if  I'm not thankful with what He has given to me. Is it wrong that I'm complaining and wanting to be more analytical and better in understanding what I'm studying? and bla bla bla.--

And this replays in my life every 2, 3 weeks or so. After complaining and expressing my wants, the same paragraph would play time and again in my head.

So, what's your verdict on this? Should we not wish for more? and just be contented with what we have? or we can continue to wish for more but couple it with investing our own extra effort to strive higher? but alas, if it's not meant to be for us, then, it'll never belong to us, no matter what you do or how hard you try~

I'm so confused. Yes, I am indeed~!!

Wallahu'alam.

Liana