You changed me, Dean. I love you... Goodbye, Dean." In other words, he confirms that he was always romantically in love with Dean.
On multiple occasions, Dean has shown that - of all the women he has slept with - he truly loves Lisa. Before and during the first few episodes of, season 6, Dean lives with the Braedens for a full year, living an "Apple Pie" life. Dean's nickname for Lisa is "Lis'".
Out of Castiel's three lovers, Meg and April, Daphne is the only one with whom Castiel had a successful relationship, as Meg was killed approximately a year later and April was a reaper tasked with killing him.
Castiel's Declaration Of Love
This turned out to be Castiel accepting that he was in love with Dean after all, and he declared his feelings for Dean as his final act. While Dean didn't reciprocate, he didn't deny anything either. In any case, Castiel's confession was enough to make Destiel a part of Supernatural canon.
6.10 Caged Heat
When the group is ambushed by hellhounds, Meg stays behind to fight them off. In need of a weapon, she kisses Castiel in order to steal his angel blade. As she pulls away, Castiel pushes her against the wall and kisses her again.
Supernatural: Former angel Castiel (Misha Collins) had a very human first on the CW hit drama: He lost his virginity to April (Shannon Lucio)! Alas, it turns out she was a reaper and stabs Cas in the chest, so yeah, no cuddling for those two.
Dean and Castiel is the relationship between the hunter Dean Winchester and the angel Castiel. Castiel was the one who freed Dean from Hell and they have become best friends.
Castiel's relationship with Sam is, first and foremost, through Dean. It is Dean who is the glue to their alliance. Sam is probably more an associate than a true friend of Castiel's. Although Sam and Cas often team up together, and sometimes even to save Dean himself, what they share is actually their love for Dean.
Although Castiel doesn't show Sam the same attention as Dean, he still cares for both brothers and develops interesting relationships with them. However, Castiel's bond with Dean will certainly live on as one of the best Supernatural pairings of all time.
The fanfic origins of the Destiel ship
Fanlore theorizes that the reason it was so popular is because the other options were a bit contentions (i.e., an incestous pairing between the two Winchester brothers, or the real people shipping of actors Jensen Ackles and Jared Padalecki).
Cass reveals that he has learned a lot about being human and reveals he had sex with April, leading the Winchester's to be surprised. Dean even goes onto make a joke about "having protection." Castiel goes to eat and Ezekiel takes over Sam saying Cass can't stay because he will bring the angels down on them.
In Castiel's final moments, he smiles through his tears as the Empty takes both him and Billie, saving Dean. Castiel's final words are "Goodbye, Dean."
The line ''I love you'', was to be directed at Cas's character and Jensen put a stop to it. Dean had no reason to say something like that to Cas, of all people. This is the response I get. Or, so Jensen just wants to stay true to Dean's character.
This is reconfirmed in the Supernatural finale when Sam's soul ascended into Heaven and materialized next to Dean's soul, making him the first person he interacted with in the afterlife. The brothers are unique in that they are the only soulmates who are family rather than spouses.
Cassie Robinson
When her father was killed, she contacted Dean for help and they eventually renewed their relationship by sleeping together, now that she saw the supernatural.
There is a canonical deep friendship between the Winchesters, particularly Dean and Castiel. He has been referred to by the boys as family on numerous occasions, and by Dean as both brother and friend.
As Metatron tells Gadreel, “He's in love with humanity.” Because Dean cares about the world, so too does Castiel. He cares deeply about the man who has fought his whole life to save others, and always chooses to save Dean, even when he won't save himself.
Cassandra Masters is the daughter of Castiel and Meg Masters and the only known Nephalem to exist, which is a hybrid between a union of an angel and demon, even being part human due of Castiel and Meg's human vessels. Cassandra acts as one of the leading main characters with Sam and Dean.
Castiel was one of the top angel soldiers in the Host of Heaven. After Lucifer killed him, God brought him back as a Seraphim, the second highest rank angel class in the angelic choir.
Atropos, Crowley and finally Castiel himself, refer to Sam and Dean as Cas' "pets".
Dean is very protective towards the people he loves. Be it Sam, Lisa, Ben or anyone else. Dean is the character which is the most betrayed. So people tend to go more towards him.
Dean is four years older than Sam and was always told from a young age to "watch out for Sammy". When a demon killed their mother, it was Dean who carried baby Sam out of the house.
Sam and Dean are unable to form romantic attachments to others, and therefore their love is locked in an eternal feedback loop, referring back only to itself. They don't have anyone but each other (and their father) to love, and since their father's death, they love none but each other.
Castiel tends to demonstrate his affection by giving you gifts - he loves seeing your eyes light up when he hands them to you, and loves the hunt of finding the perfect gift; a very different hunt to what he's used to.
After 15 seasons and many long years of pining, filthy fanfiction, and debating fellow stans, Supernatural finally made Destiel canon.