Watch the video. It explains how Destiny actually makes use of operant conditioning to encourage you to buy each DLC and expansion.
If you are unfamiliar with the techniques of operant conditioning, here is a link to explain it in some detail.
http://m.simplypsychology.org/operant-conditioning.html
Edit: Thank you to Bant Spider for the video.
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This is nothing new, MMOs are based on this and their replayability is a function of this, it makes grinding less grindy, repetitive actions more rewarding and the overall structure of the game more engaging, especially because of social pressures (i.e. everyone including your friends are doing it). But Destiny is not as bas as other MMO skinner box games, i mean look at Star Wars: The Old Republic