With McGann, it was only barely hinted at with Daphne Ashbrook. So, with Tenant/Eccelston and Rose, it was still fresh and new, so their Love story was OK. Bringing in Martha with her lust for the Doctor was old hat and annoying (Though I lik Martha other than that). Donna was against Hanky Panky in the TARDIS, and instead of Amy, he went her Daughter. A hookup with Clara is just very wrong considering his Marriage to River, and the Love with Companion thing is definitely getting old now.
It happened twice and it's already getting old? Think about how many Dalek stories we would have missed if it worked like that!