"Our species were designed for efficiency. We're not burdened by the emotional imprints like humans."
Humans experience human emotion by interacting with biochemical (internal) and environmental (external) influences. Examples of Human emotion is love, vengeance, fear, sadness, happiness and joy and loads more.
Although V's are not related to humans and are from a different planet, they can actually have the ability to experience human emotion when they are in a human environment. Visitors sent to Earth prior to the Visitors showing themselves to humanity known as Sleeper Agents, which some have experienced human emotions.
Before the V's showed themselves to Humans, they became aware that V's can experience human emotion. When Ryan arrived to Earth to assasinate John May, he told him that John May is sick because he has human emotion, to which John May found Bliss a sickness and human emotion beautiful. When John May was hiding away from Ryan for a year, Ryan had no choice but to stay on Earth and look for John May, during that time Ryan experienced human emotion.
According to Anna, the Visitors were designed for efficiency, they are not troubled by emotional imprints like humans, and so V memories are information kept, whilst human memories contain emotion, not just information.
When Anna found out that a suicide bombing happened on the Sydney mothership during her Bliss, she came to the conclusion that the Fifth Colunm is immune to her bliss and can experience human emotion. She ordered Joshua to setup an empathy test to "find out who's Fifth Column, and who isn't" When Anna learnt that there was a margin of error in the empathy test, which meant that there were V's that weren't Fifth Colunm, but did experience human emotion. Anna brushed it aside, and later gave the ones who failed suicide pills, to which those who took them were loyal to her.
When Joshua was forced to skin a friend of his, he was showing grief, but when Anna was giving out her Bliss, Joshua found it difficult to fight the effects. Also when Ryan was separated from Valerie, he found it difficult to fight of Bliss. This shows that V's who are sad can actually recieve bliss and can then return to having no human emotion.
Lisa was another Visitor, who experience human emotion by being with Tyler. At first he was an assignment, Lisa may develop genuine feelings for Tyler. When a upset human who had a "nightmare" of needles being poked into her, and Lisa showed some sympathy to her, which made Anna concerned. When Anna was deciding which of her condonlense speeches to give, she asked Marcus which one to which he found them all sufficient, and asked Lisa to which she chose the one which was more sincere. Anna replied "Your grasp of human emotion is... impressive." which probably made her suspicious or make her realize that she has human emotion.
When Anna's eggs were destroyed, she experienced her first human emotion which was vengeance, and activated the second stage of the Visitors plan prematurely.
In Red Rain, the captains of the 29 ships were surprised by the sudden and unexpected release of the Red Sky, and were concerned there own queen was infected by human emotion. She then wanted a holo-conference with the captains, only to be shocked they are on-route to meet her in person. During the meeting, she hacked a V captain with her tail, which was enough damage to rip open his human skin and reveal the V flesh, exclaiming "I may wear this skin, but I never forget what we are beneath", and later killed the captain.
When she was in the room with the remaining six V soldiers, which the other six died. She then goes to one of the V soldiers and holds it's hand, surprised by it's actions. Marcus revealed that these six survivors will be permanently flawed. Knowing the death of the V eggs triggered her emotional outburst, Anna then pulls the plug on the remaining six saying before that she will never again show emotion, she exits the room trying to ignore the screams of the six survivors and showing sadness. She then afterwards confronts Diana for either to help her get this 'infection' away from her or something else.
Before Anna was the Visitor High Commander, Diana was in control and wore human skin and over time Diana herself experienced human emotion, which Anna felt a threat to the Visitors and so took over, before it was her time to be in the crown, and imprisoned her on a part of the ship which is a replica of the Visitor homeworld.
By the way Anna words it, "infected by the human skin" it's probable that the V's reptillian skin cannot feel anything and so they are never going to feel happiness, sadness or any other form of emotion, but when the human skin is applied, the V's have the ability to feel things and so therefore experience human emotion. But another theory is that on the V homeworld, they've never ever in their history come across emotion and when placed in another species environment which has emotions, it's possible therefore that the V's can experience it over time.
Another possible feeling as to why V's do not feel human emotion, is probably because there civilisation is not on a planet but on 538 ships plus the 29 motherships. This is supported when John May says to Ryan about feeling the wind in your face which supports that the V's only live on ships. This therefore makes a V not experience human emotion, and makes it likely since V's can be susceptible to carry human emotion.
In Devil in a Blue Dress, Joshua announced that he may not be able to solve the problem of human emotion by the time the begin inter-breeding. Anna wondered as to why Amy accepts the bliss because of the fact she is half-human and half-Visitor and yet the human side does not reject it. It turns out the V brain accepts the bliss at a frequency that isn't rejected by her human side, because of Anna's private phosphorus supply was stolen. It was concluded theoretically that there is form of bliss that can work on humans. It worked, but with disastrous consequences. The reason she is focusing on bliss, is that if the problem of human emotion cannot be solved by the time of inter-breeding then Anna can give bliss to the next generation of the Visitors to prevent human emotion from happening.