Sherlock:
Believer by Imagine dragons. I thought it was appropriate because you always believe in him and "pick" up his broken heart when he isn't feeling well or when he feels like turning to drugs.

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John:
Stand by you by Rachel platten
I thought this was the song because you and John are always there for each other through it all. And you know that he'll stand by you through it all.

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Mycroft: can't help falling in love with you (cover) by Twenty one pilots

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Moriarty: run away by Galantis

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Greg: In the name of love by Martin Garrix 

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