Uranus and Neptune are probably the most loyal and devoted senshi in the solar system. Their pride and confidence in their purpose have served them well, enabling them to resist some of the greatest of evils. The destined couple holds fast to their beliefs and are willing to sacrifice their lives to ensure the fulfillment of their duties.
The main purpose for Uranus and Neptune is to protect the princess (by stopping the approaching silence). And they aim to complete that mission at any cost. A good example is Episode 106 of the anime where Haruka and Michiru first meet. Both understand the danger of the "impending silence" and know they are the only ones who can stop it. At first, both are very reluctant to accept this duty - each has their own separate dreams and passions. Haruka, especially, tries to run away from her fate and follow her dream of becoming a motorcross racer. But once Michiru appears, she can no longer fight against destiny. Likewise, Michiru had to give up her dream of becoming a professional violinist in order to "save the world."
In the manga, there's a slight change, though. Haruka is a successful and popular motorcross racer and Michiru is an esteemed violinist. Rather than give up these dreams, they both use their skills as resources to get them closer to fulfilling their duty. Their skills as performers and their "high-class" status enables them to attend the prestigious Mugen Gauken (Infinity Academy) and infiltrate its staff and student body.
However, both versions stress the loneliness Haruka and Michiru must endure. As senshi, they come off very cold and harsh - refusing to help the other senshi unless it furthers their own progress. Revealed in both Episode 91 and Act 27, Uranus and Neptune clearly state thet they "have no intention of fighting together" with the Inners. They even attack them in the manga! But they don't intend to hurt them at all. Quite the contrary, these two do not want to endanger the Inners in their mission.
Haruka and Michiru understand the dangers of their mission. They were give powers stronger than that of regular senshi in order to protect the world from the greatest of evils. That means a lot of fighting, pain, and worry falls on their heads. And they don't think the Inners can handle it. Or rather than don't want the Inners to handle it. They are willing to take all the problems and all the blame to protect the Moon Princess and the Inner Soldiers. It's kinda easy to understand where they're coming from; if the Inners are destroyed, then there's no one else to ensure the safety of the Princess and the world will be destroyed. Even though they are more powerful, both respect the importance of the Inners and will do anything to keep them alive.
Also, to keep their minds focused on their mission, they push everything else away unless it helps them. I think this is mostly why they appear to be very mysterious and "lonely" when compared to the others. They only friends they seem to have are the Outers (and eventually in the Inners) at first because of their duties. It would not be wise to get attached to the people of the earth because they may have to sacrifice themselves or humans to guarantee the success of their assignment. Feelings evoke fear, fear evokes panic, panic evokes ruin.
But the fun doesn't stop there! Not only are they willing to risk their lifestyles and their social lifes, but they will glady give up their own lives. The unveiling of the talisman in Episode 110 expresses this idea passionately. Haruka herself promises to "make any sacrifice and use any means to get the talisman." At the Marine Cathedral, 
Neptune reminds Uranus to "concentrate on the goal and not worry about each other's safety." Both care deep about each other but are more devoted the mission at hand.
There are times, however, where their deep love will get in the way of their mission and may stop it. In the same episode, Uranus was about be attacked by Eudial, but Neptune diverted her attention and took the blow herself. Even though she lost her heart (which turned into a talisman), her love was safe - for the time being. Distraught and confused, Uranus thinks about the sacrifice her partner made for her and decides to do the same and give up her talisman. It's a very emotional and heart-wrenching moment when she pulls the trigger and falls dead to the ground. But all this for the mission; all is for the safety of the world.
It is later revealed that this constant self-sacrifice for love and devotion is their "style of fighting." Most prominently at the end of the Stars season, the destined couple risks their lives - and their honor - to destroy Galaxia. In Episode 197, Uranus and Neptune give up their starseeds to Galaxia and join her as her "loyal"soldiers. They prove their worth by killing Pluto and Saturn as Usagi and the Starlight watch. They continue to serve as Galaxia's puppets until the perfect chance arrives. When Galaxia orders them to attack the remaining senshi in Episode 198, the destined couple betrays her and tries to take her starseed; but, they fail and they lose Galaxia's power. Even at the cost of their reputation as protectors of the solar system, Uranus and Neptune will give up their soul to have a chance to save the world. Galaxia was even impressed by the drive and loyalty of these senshi - and she's their greatest enemy!
I can only admire their stubborn devotion and faith in their duties because, I'm not ashamed to say, I don't think I would be that dedicated in a situation like that.
[[ introduction ]] ~~ [[ separation ]]