Yellow is such a strong, vibrant color that brings people's attention into this idea of pure happiness within it. I can think of no other color that subjugates such a positive emotional response as yellow does. Yellow, as any other color, comes in many different shades and tints, all which make the brain's response different while still imagining happy days.
The color yellow is seen in history, feng shui, modern culture, and my own take as being a color to signify happiness. When I was researching the "history" of yellow, I found a very complex way of saying what yellow stands for. Basically, the article said yellow comes from the Proto-Germanic word "gelwaz" which is based on the word "gel" in Indo-European meaning bright and gleaming. Took a while and they beat around a very old confusing bush, but yellow has a meaning we still see today in our culture. Feng shui, a secret love of mine (but I can never figure out how to "feng shui" my life), uses colors to signify various portions of your life and what inspires each one.
A strong yellow, for instance, is a Fire element and is used in offices as energy for career projects and balance and it suggests using it in the South bagua of your home. A light yellow color is considered an Earth element which adds stability, nourishment, and calm protection over your relationships. In modern culture, or design you could say, yellow seems to always be used to brighten up dull or dim spaces.
Personally, I see yellow as an encouragement. You see yellow in nature in the sun, sunflowers (my favorite), lemons, and various other natural things. All of the things listed invoke a happy or cheery response. When I see sunflowers, I immediately perk up, I visualize sunflowers as a reminder to always look for the good because they always look to the sun. Lemons, a great dessert flavor in my opinion, are adorable little fruits that pack a big punch on their own, like the color they are.
Yellow stands for better days and better things to come soon. You never feel sad around this color, which is a plus, and you never feel like you couldn't possibly be happier. My personal favorite color of yellow is the lemony-golden-bright yellow. It seems like the most effective without being too cheesy (see what I did there?).
Lemons themselves stand for ideas as well. The quote, "When Life gives you Lemons, you make Lemonade" is truly one of my quotes. I personally love the first part the most, because the end can vary so much for who you are and how you view things, it's all up to you. Lemons are usually remembered as that sour, pucker your face up flavor that is not reminiscent of happy times, but the sweet side of it, oh how summer days are defined by them.
I love the color yellow so much because in my mind, it means positive things and happiness. It's a reminder that everything is about looking to the good and letting the shadows or bad fall behind. So be that sunflower, be that sweet memory of lemon, be you because that's all we could ever ask. 
