Why is the Sicilian Defense considered hard to play? I have been told never to play the Sicilian. It is a very complicated opening that even grandmasters aren't ever able to analyze and play well enough. But why, I ask?
Good heavens.  Play it!

The Sicilian is not a defense for the lazy or defensive player - the Black side will need to be booked up, tactically sharp, and more than anything will need to understand why it works, and how to make it work.

Regarding that GMs can't play it, that's nonsense.  It's a deadly tool in their arsenal.

This opening's hypermodern ideas eschew some well-established 'rules' including immediate control of the center.  It encourages an asymmetric game that reduces the chance for a draw.  In many variations, black sacrifices the exchange on c3 in order to gain time and break white's center.

In the Sicilian, black frequently loses quickly or wins long.

Here's an example from 2013 of Anand with the black side taking apart 2716-rated Arkadij Naiditsch.  According to 365chess.com, this is an ECO B52, the "Canal-Sokolsky attack". As you can see from this game, the Sicilian may not be the best choice for the timid.


[fen ""]
1. e4 c5 2. Nf3 d6 3. Bb5+ Bd7 4. Bxd7+ Qxd7 5. c4 Nf6 6. Nc3 g6 7. d4 cxd4 8. Nxd4 Bg7 9. O-O Nc6 10. Nde2 Qe6 11. Nd5 Qxe4 12. Nc7+ Kd7 13. Nxa8 Qxc4 14. Nc3 Rxa8 15. Bg5 e6 16. Re1 Nd5 17. Nxd5 Qxd5 18. Qxd5 exd5 19. Rad1 h6 20. Bc1 d4 21. Rd3 Rc8 22. Rb3 b6 23. Kf1 Ne5 24. Ra3 a5 25. b4 Rc2 26. bxa5 bxa5 27. Rxa5 Nd3 28. Ra7+ Kc6 29. Rxf7 Nxe1 30. Kxe1 Rxc1+ 31. Kd2 Rg1 32. Rxg7 Rxg2 33. Ke1 Rxh2 34. Rxg6 Rh1+ 35. Kd2 h5 36. Rh6 h4 37. a4 h3 38. a5 h2 39. a6 Kc7 40. Rh7+ Kb8 41. Ke2 d3+ 42. Kd2 Ka8 43. Rh5 Ka7 44. Rh6 d5 45. Rh8 Kxa6 46. Rh6+ Kb5 47. Rh8 Kc4 48. Rc8+ Kd4 49. Rh8 Ke4 0-1


Just for fun, here's Tigran Petrosian, the Patron Saint of Positional Players, dismantling Boris Spassky.  This is a B42 Sicilian, Kan variation.  (I like to mention such games because it's easy to get into the habit of thinking of some players as purely positional, or some as purely tactical players.  All GMs are competent in all aspects of the game.)  

It's the 1st game of the 1969 world championship.  Again, we see a long game here.  Petrosian would lose the black side of a Sicilian Dragon to Spassky later in the same match, and it would be over in under 30 moves.

[fen ""]
1.e4 c5 2.Nf3 e6 3.d4 cxd4 4.Nxd4 a6 5.Bd3 Nc6 6.Nxc6 bxc6 7.O-O d5 8.Nd2 Nf6 9.Qe2 Be7 10.b3 O-O 11.Bb2 a5 12.f4 g6 13.Rad1 Nd7 14.c4 a4 15.f5 exf5 16.exf5 Bf6 17.Bxf6 Nxf6 18.Qf2 axb3 19.axb3 Ra2 20.fxg6 fxg6 21.h3 Qe7 22.Qd4 c5 23.Qf4 Bb7 24.Rde1 Qg7 25.Qe3 d4 26.Qe6+ Qf7 27.Qe2 Re8 28.Qf2 Rxe1 29.Qxe1 Qe8 30.Qxe8+ Nxe8 31.Be4 Rxd2 32.Bxb7 Nd6 33.Bd5+ Kg7 34.b4 cxb4 35.c5 Nf5 36.c6 Rc2 37.g4 Nd6 38.Rf4 d3 39.Rd4 d2 40.Bb3 Rxc6 41.Rxd2 Ne4 42.Rd7+ Kf6 43.Rxh7 Rc1+ 44.Kg2 Nc5 45.Bf7 b3 46.g5+ Kxg5 47.h4+ Kf6 48.h5 Rc2+ 49.Kf3 b2 50.Ba2 gxh5 51.Rxh5 Rc1 52.Rh6+ Ke5 53.Rb6 Na4 54.Re6+ Kd4 55.Re4+ Kc5 56.Rxa4 Ra1 0-1