This is an issue which has almost haunted the previous two incarnations of the Doctor: the awkward relationship he develops with his female companions. Tennant had the heartbreaking separation from Rose, with great implications of unrequited love. Whereas Smith had the bizarre love triangle between Amy and Rory, not to mention the love-unspoken relationship with Clara. Yes, there have been companions where romance wasn't an issue (Martha and Donna, for example), but these seem like mere blips after the attention the others received because of the apparent romantic links. The writers have explored it from a number of ways, whether it be the Doctor and his companion loving each other without doing anything about it, or whether his companion is hopelessly in love with him, but it all comes down to the same thing: the Doctor never settles with a companion. For one, it would spoil the format of the show, and for another, she's bound to age much faster than he is... Hopefully the return of an older actor as the Doctor will make it more difficult for the writers to make his inevitably young and attractive assistant to feature a love angle, much in the mold of the Eccleston era.
