However, there are two technical terms you need to know: "major" and "minor".
Although they have a strict technical definition, the nature of major chords make them sound happy. They should put pleasing thoughts into your head.
If you count the number of keys on the piano (including the black ones!), there will be four keys between the bottom note and the middle note, and three keys between the middle note and top note.
On the other hand, minor chords sound sad. By changing just one note in a major chord just very slightly, your happy thoughts have turned into depressing thoughts. Minor chords have three keys between the bottom note and the middle note, and four keys between the middle note and top note.
A Major 7th occurs when you stack a minor chord on top of a major chord! Because you have a mix of both major and minor chords, it can create a mood of confusion or mixed feelings.
The ear tends to hear the higher notes in a chord more prominently than the lower notes. Based on this, you can illustrate various emotions by stacking chords accordingly. For example, in the song "You Look So Fine" by Garbage, an E minor chord (sad!) turns into a hint of happiness by adding a C-natural to the bottom of the E-minor chord. This changes the E-minor chord into a C-major 7th - the top three notes are still in E-minor (and the most noticeable to the ear), but the bottom three are in C-major (which, remember, sounds happy!).
Unfortunately for the listener, the song eventually returns to E-minor, not leaving you any happiness as the album comes to a close.
Similarly, the song Dogs by Pink Floyd opens with a series of Major 7ths. This time however, an extra note is occasionally added to the bottom, giving us a major chord in the middle, surrounded on top and on bottom by minor chords!
This sound is very well-suited for the song. Although the dog itself can be represented happily with the major chord in the middle, there really is no happiness to be found in this lifestyle, and the surrounding minor chords emphasize this.
I'm not entirely sure what the point of this post was, but I feel it describes me well right now. Take from it what you may.