Both those sisters have had to put up with an unbelievable of crap. I wish I could say it beggars belief, but sadly , it doesn't. They are incredibly talented and dedicated and that should garner respect, but in many cases it has just garnered them abuse.
Andy Murray gets something similar from the UK press, albeit in smaller doses. The similarity is I think because neither the Williams sisters nor Andy Murray "know their place". That is, they don't try desperately to conform to the standards set by an entrenched hierarchy.
So they don't cowtow to oppression. That is something, that, in my opinion, makes them even more worthy of respect than their sporting achievements. They haven't changed themselves.
I think that may also be part of the reason they have been so successful- they haven't wasted time on bending over backwards to be accepted by an establishment that tries to force them to justify their very existence.
Incidentally, why should any sportsperson be cheery? If that's their personality, then fine. But if it isn't, it isn't.
Serena and Venus reportedly grew up playing on public courts knowing to drop to the ground when they heard gunshots. It takes a lot of determination to persevere in those conditions. It is scary for people in the establishment to think that sheer talent, drive and determination can overcome disadvantage. It shows that all the privileges and advantages people in the establishment try so hard to keep for their children, and their children only, can crumble in the face of talent, discipline and single mindedness.
Andy Murray has latterly been quite open about the fact that a lot of his motivation to do well in tennis was to honour the memory of his schoolmates who died during the Dunblane shootings. If that's where he is coming from emotionally, why on earth would he be the on-court reincarnation of Henri LeConte? And none the worse for it.