Oh, sure, they have plenty of reason to be wary. But looking at all humans as enemies is not a good idea. Letting them prove themselves, sure. Distrusting them and watching them closely until they have, even more so. The PC can easily be a racist jerk. But the elves often seem to refuse to accept help from friendly humans simply because they are human.
Honestly, I don't think that's necessarily the case with every clan. All the clans are different, and their treatment of humans varies. We have a Dalish clan who adopted a human infant who was abandoned by her child (who became Aveline the Knight), we have the clans signing a treaty to aid the Grey Wardens in protecting human civilization during the Blight; in one clan, we have Zathrian willing to honor the treaty if The Warden can end the curse that has crippled half of his clan with lycanthropy, we have Elora willing to accept aid in helping the halla, Athras is willing to accept help in finding out the truth about Danyla, and even the young Cammen is willing to receive assistance with his romantic endeavor for a young woman he's in love with.
It doesn't seem often that a "friendly" non-Dalish encounters the clan. It's likely centuries of hostile encounters with Andrastians attempting conversion to the human religion, mobs of angry humans trying to drive the elves out, or templars hunting mages. The protagonist seems to be the exception to the rule, which is why the demeanor of Zathrian's clan changes once The Warden starts helping Dalish in need (thereby providing that he or she is a friend).
What grated on my nerves the most reading TME, was not only that Briala quickly turned to seeing humans this way, and she couldn't have cared less about the welfare of human commoners who, at best, were slightly better off than the elves. They can play the arrogance card all that they want, but at the end of the day, the elves won't get a think without collaborating with humans.
To be fair to the character, we also have Celene burning down a plethora of elven men, women, and children to secure her standing with the nobles, who saw her as too soft on the elves (given the heretical play about her and Briala adding fuel to the fire), and Briala was willing to accept that decision. Briala was also deeply in love with Celene, a human, and willing to take risks that endangered her life to help her; she seemed to be in a tough situation where she saw an opportunity to help free her people from a system where they are subservient, and seized it. After all, until Briala realized the role Celene played in the tragedy of her past, she was willing to work for Celene towards the goal of ending the civil war and securing Celene's role as Empress.
As Briala says, “I will fight for the others who have no one to champion their cause.”
Given the elven historical account about the fall of the Dales and Halamshiral being burned down recently, there's reason for elves to be wary about collaboration with the shemlen. I suppose that will be a task left to the Inquisitor. I'm still curious how the narrative will handle a Dalish protagonist being found by a pseudo-Chantry organization (Cassandra and the fledgling Inquisition who helped her interrogate Varric in Dragon Age II), and hopefully there will be a variety of options and responses to provide.
They're best chance in DAI will likely be allying with the Inquisition, a predominately human organization with a potentially human (and likely not elven) leader.
I hope allying the Dalish clans with the Inquisition is a possibility with my Dalish Inquisitor, since it seems possible to do so with the mages or the templars.