It's obvious that Moriarty has known about Sherlock since they were kids (the case of the drowned boy - Sherlock's first ever case that he wasn't able to solve) but why does he approach him now 22 years later. Sherlock had know idea who he was and I think he would have been happy having no idea who Moriarty was. He was the one who put himself on Sherlock's radar (through the cab driver).
I have a few theories as to why Moriarty is now targeting Sherlock.

1) He's Angry at Sherlock - As we all know Sherlock has an obsessive personality and he was obsessed about the case of the drowned boy and he was close to solving it but didn't have all the pieces so he gave up. I think that angered young Moriarty because this little genius had found his counter part and he just gave up on him he wasn't good enough for the great Sherlock to obsess over. So he created an evil empire became a Consulting crimanl and now that he was on top of the world he would take Sherlock down.

2) He was bored - Like Sherlock he is also an adrenaline junkie and he basically has the criminal world at his feet none could stop him, none even came close except for Sherlock and he wanted to go up against someone he felt was his intellectual match and like Sherlock he is/was willing to die to prove his intelligence.

3) Moriarty was dying - I felt like Moriarty had the criminal world at his feet, he had everything and he was happy but he had like a brain tumour ( like the cab driver) or something and he wanted to go out on a high by defeating his long time enemy Sherlock(unbeknown to the latter), because I felt like Moriarty put way too much effort into the destruction of Sherlock for him just being bored I feel like he knew he wasn't going to live through it and you know what they say "Go big or go home". And technically when Moriarty shot himself in the head he was winning, if he is in fact dead, he died haven beaten Sherlock.

These are my theories please tell me what you think.