Well u know ... by the time of ROTJ, he's old, over the hill ... and ... more importantly ... its his son u see ... in seeing his son, suddenly there are tremors of the sleeping ghost of anakin within vader ... i quote " There is still good in you ...... that's why u didn't kill me then ... that's why u won't take me to your emperor now " ... so basically, he wasn't totally merciless against Luke as was his usually wont.
None of the afore mentioned problems would be there against Dooku.