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 isn't quite news to me and the video articulates it very well. Even knowing this I'm not swayed in my decision to play it. It's a fun game, and I enjoy the mindless grind. I DO think this is stuff a lot of people ignore and should think about so hats off to you for posting this, and send my regards to the gentleman who made the video. Good job.