Anakin didn't have to be exactly like Luke, but it was established in Episode I that Anakin was a good kid, and for whatever you think of Jake Lloyd, he is a good wholesome kid that you do sympathize with. AOTC Anakin is a jerk the minute he is on screen as he argues with Kenobi in front of Padme after whether they should protect her or find the killer, and from that moment on I just never really liked him, let alone cared for his plight. So when you have these tragic events happen to him in ROTS, you are almost rooting against him and wanting him to fall into lava as I sided with Kenobi at the end of the movie. Anakin should have been a likeable character until he turns, then you have that contrast that would make you feel his plight, cause you could understand what he was doing.
A good example of a tragedy would be Luke or Han turning on each other at the end of ROTJ, and watch them fight as trying to figure out who to root for? You wouldn't, you feel for the one that turned to the darkside, cause you cared about that character for 3 movies. I look at the Anakin character after Episode I, and sometimes wonder what the hell Lucas was thinking.