Sven is Kristoff's pet and best friend. They have known each other since childhood and are inseparable. Kristoff has a very strange habit of speaking for Sven in a silly voice, but Sven doesn't seem to mind. Kristoff loves Sven a lot, and always make sure he has enough food to eat (carrots), even though they share their food with each other most of the time. Kristoff can understand what Sven is saying, which is why he does voice-overs for Sven, and Sven often acts as his conscience, being the one to convince Kristoff to continue accompanying Anna on her journey so she wouldn't freeze to death and could give Kristoff a new sled. Sven was also smart enough to realize Kristoff's feelings for Anna, and tried to convince Kristoff to confront his feelings for her. When Sven and Kristoff were rushing to get to Anna, Sven accidentally fell into the river. Despite needing to get to Anna immediately, Kristoff looked back and waited a few moments to make sure Sven was alright before he rushed back to save Anna, indicating how much he cared for his reindeer.
Kristoff seems protective of his friend as when Anna and Sven approached the end of a cliff and she told him to jump, Kristoff said she couldn't tell Sven what to do and then proceeded to tell Sven to jump. He also told Olaf not to talk to his friend demeaningly, even showing hints of jealously towards others who make an attempt to befriend him.
Though Sven can at times be annoyed by Kristoff's selfishness and Kristoff hates it when Sven acts as his conscience, Kristoff and Sven are very close and treat each other more like equals and siblings rather than a traditional master/pet relationship, with both caring for the other's happiness and well-being.
After Kristoff, Olaf is Sven's closest friend. The two are regularly seen together, specifically in promotional and spin-off material, often taking part in comical side shenanigans. Their friendship can be easily explained by their similar personas; both being childlike, affectionate, curious, and rather playful. Upon their first meeting, Olaf immediately took a liking to Sven, finding the reindeer and his antics to be adorable. Likewise, Sven grew an immediate fondness for the snowman, mostly for his whimsy and carrot nose.
Throughout most of the movie, a few scenes made it seem is if Sven was only interested in Olaf because of his carrot nose: as carrots are the reindeer's favorite snack. However, in the end, after successfully snagging the nose, Sven immediately returns it, showing he was merely teasing the snowman, and their friendship is proclaimed.
Kristoff may have a hint of jealously for the duo, as seen after the chase with Marshmallow, where Olaf affectionately snuggles Sven's muzzle, to which Kristoff reacts by grumpily separating the two.
Since the beginning, Sven had a liking for Anna, immediately claiming her as a good friend once the journey to find Elsa began. It was Sven who, in a way, convinced Kristoff to continue helping the princess on her quest, knowing it'd be a dangerous journey and that help was necessary. This also introduces Sven's protective side for the princess, as he's often seen rushing into action to protect her, specifically during the third act of the film, where Anna is at risk of dying from a frozen heart. It should also be noted that during the third act of the film, the moments in which Anna is dying of a frozen heart, Sven's usual goofy side is absent, and his determination, valiant ways, and noble stubbornness are the only traits showcased up until Anna's safety is secured and the curse is broken. This, once again, proves Sven's unconditional love for the princess.