I was going to write a fanfic based off another image centered around Rarity discovering an older Spike’s interests in crossdressing and hypnosis, but since then I’ve been too busy to write and my energy goes into other projects. However with this pic I’ll give a smaller, different version of it:
 
As Spike grew into adulthood, he discovered he had some interests that weren’t exactly normal. He loved the idea of being hypnotized into a girly dragon slave for his crush, but he kept it a secret, that is until Rarity happened to catch him trying on one of her swimsuits. The encounter brought Spike to tears as he revealed his secret to Rarity, but Rarity was very open minded about his bizarre fantasies. Having spent enough time in bigger metropolitan areas with many different perspectives, Rarity knew these sorts of fantasies were perfectly normal and she was happy to be Spike’s confidant. This was a great relief to Spike since he had kept these fantasies completely hidden into adulthood and it had been weighing on his mind. Which brings us to what we see here: Rarity decided to make Spike some articles of clothing for him to indulge his fantasies with. So now Spike is marching in private as the earbuds command him and enjoying some private time as the hypnotized girly dragon slave he likes to become when he’s alone.