Well, for starters because we are probably( i hope) going to win the league, so we are going to have CL next season, something that he is absolutely crazy for, have a WC manager in Conte, a few WC players like Hazard,Courtois,Azpilicueta and Kante, are based in London,and judging by pretty much all the reliable reporters, are going to go on a really big spending spree this summer, so maybe he wants to be a part of that?
Plus you know, he also just happens to have supported Chelsea as a club since he was a kid.
I get that you dont want to see him go, and by no means is this over, but there is plenty of reasons for him to want to join Chelsea, although there is also a few good reasons for him to stay at Everton, pretty young team, so could be poised to push on, and maybe join the "elite" of the PL, being the main man for the club, something he probably is never going to be at Chelsea with Hazard there, so there is plenty of variables on his decision.
But Matt Law has some serious inside information at Chelsea, he is pretty much the go to guy, when it comes to Chelsea, so he definitely didnt made this out of thin air, if he said that, there is in all likelihood some truth to it, but this story definitely still has quite a few twists and turns before it is concluded, but like you guys, one way or another, i hope this is over at the beggining of the summer.
Anyway, this is just my input on the situation, hope you guys dont mind.
Cheers.