I've also noticed the same "all-happy" feeling when people enjoy something I've cooked. For instance, I had a late birthday party at my house a while back, where I made chocolate chipotle cakes. They turned out delicious, if I do say so myself, and a large part of my enjoyment of the evening came from having made something that my friends enjoyed.
Why do you think such actions, and in general, doing nice stuff for other people, engender such feelings? Allow me to answer my own question and pontificate a little bit.
- In the case of cooking, I think at least part of it is the inherent satisfaction of taking a bunch of ingredients and creating something better. I imagine this is what artists feel when a painting turns out, or how musicians feel about composing or performing a great piece of music. Even in buying a gift, we take our fluid funds and convert them into an object of some sort. As humans, I think, we want ordo ab chao - we strive to create. I can't help but think that this desire is God-given, and reflects our potential as children of God.
- Part of the satisfaction of giving a great gift is its emphasis on the relationship you have with the recipient. Not only do you know them well enough to want to give them a present in the first place, but you also know them well enough to know exactly what kind of thing they would want. Seeing the recipient's happiness is a confirmation to you that you know them well - that you guessed right! And then you're even more happy because not only have you done something good, you've done it well.
- I don't think it's a coincidence that I started pondering this right before I came across Mosiah 2:17: "And behold, I tell you these things that ye may learn wisdom; that ye may learn that when ye are in the service of your fellow beings, ye are only in the service of your God." Serving God brings happiness. Serving our friends, through gifts or cooking or whatever other means, is serving God. Therefore, serving our friends brings happiness. That's right, I just threw down a categorical syllogism to make my point.
What do you think? Tell me your ideas in the comments. Or spam in the comments, but that'll get deleted.