Love

Love was an emotion which K9 Mark 2 struggled to understand, receiving no clear answer when asking his human company. (TV: Aeolian) Peri Brown defined it to Yrcanos, the man who would become her husband, (TV: The Ultimate Foe) as "when you care for someone or something more than yourself", "sometimes more than life." (TV: Mindwarp)

The First Doctor noted pride as an example of emotions to the Cyberman leader Krail, along with hate, pride and fear. He questioned, "Have you no emotions, sir?" (TV: The Tenth Planet, Twice Upon a Time)

The Doctor's love life
Although River Song once declared her belief that the Doctor was above the capacity for romance (TV: The Husbands of River Song), he has proved himself capable of expressing romantic feelings for others.

When with the Aztecs, the First Doctor accidentally got engaged to a woman called Cameca, when she poured cocoa beans in front of him, not realising the Aztec symbolism that sharing cocoa was a proposal of marriage. Cameca gave him a brooch during his visit, which he was planning to leave behind before leaving in the TARDIS, but then picked up again, indicating that he did show real affection for Cameca, possibly even romantic feelings. (TV: The Aztecs)

The Doctor didn't display any romantic suggestions towards his companions throughout a number of his incarnations, and his relationships between him and them are generally aren't extended past the platonic.

The Sixth Doctor once confessed that be believed he was falling in love with Bianca, an evil future incarnation of Iris Wildthyme, though these feelings were ultimately a ploy by the Doctor to defeat her, as well as an effect of the psychic worms in the alcohol Bianca had given the Doctor. (AUDIO: The Wormery)

The Seventh Doctor once stated that he creates an external image of asexuality so as to keep his relationships with other simple. (PROSE: Death and Diplomacy)

On his regeneration into his eighth incarnation, the Doctor began to explore the realms of romantic connections with others, involving sharing a kiss with Grace Holloway (TV: Doctor Who). Even though he admitted to I.M. Foreman that he felt that he couldn't pursue sexual relationships with his companions, it's is strongly implied by Bernice Summerfield that she and the Doctor participated in a sexual encounter. (PROSE: The Dying Days, AUDIO: Benny's Story). After travelling with the Eighth Doctor for several weeks, Charley Pollard had realised she'd fallen in love with him. (AUDIO: Letting Go) She confessed these feelings to him when she felt she was going to die (AUDIO: Neverland), and the Doctor confessed that he loved her as well, but their relationship remained platonic. (AUDIO: Scherzo)

There were strong implications of a romantic connection between the Ninth Doctor and Rose Tyler. The Doctor was reluctant to confess anything of the sort, though many others mistook him and Rose for a couple. In one instance, a Dalek exploited the Doctor's repressed romantic feelings for rose to make him compliant. (TV: Dalek) The Doctor's feelings for Rose were all but confirmed when he kissed her to absorb the time vortex from her head into his own, destroying the cells of his body and forcing his tenth regeneration. (TV: The Parting of the Ways)

The Tenth Doctor had a lot of female admirers, including Queen Elizabeth I, to whom he got married and left behind at the alter (TV: The Day of the Doctor). He once fell in love with Madame de Pompadour, who took him away to "dance". He was particularly saddened by her death before she had the opportunity to travel with him. (TV: The Girl in the Fireplace) Rose was obviously jealous of the Doctor's relationship with Madame de Pompadour and Sarah Jane Smith (TV: School Reunion). When saying goodbye to the Doctor, Rose admitted that she loved him, a feeling that the Doctor was unable to reciprocate before the message is prematurely cut off before he could finish what he was saying.