Most of the time in Tolkien's works, the Free Peoples of Middle Earth keep to themselves. Elves hang out with Elves, Men with Men, and Dwarves ignoring as much of the rest of the world as possible. But every so often there are instances of different people groups mingling with each other, and this is always a Good Thing.
Gondolin was the greatest of the cities of the Elves. One of the marks of its greatness was that in was inhabited by both Noldor and Sindar. And when Hurin and Huor, and later Tuor, came to Gondolin, they were treated as valuable members of the community, even though they were Men. At the Gates of Moria, Gandalf wistfully recalls a happier time when the Elves of Hollin were in close friendship with the Dwarves of Moria. At the beginning of the Hobbit it tells of a time when the Dwarves of the Lonely Mountain and the Men of Dale lived in close friendship before Smaug laid waste to the countryside. And by the end of the book, that friendship has been reformed with the added close allies of the Elves of Mirkwood.
All the great love stories of Middle Earth are romances between the races: Thingol and Melian, Beren and Luthien, Tuor and Idril, Aragorn and Arwen. And one of the great relationships of The Lord of the Rings is the friendship between Gimli and Legolas.
But the best example of mingling is embodied in the Silmarils. The Trees of Valinor were among the greatest of the works of the Ainur, and Yavanna's masterpiece. But they were at their most beautiful when the golden light of Laurelin and the silver light of Telpirion mingled. Feanor, the greatest craftsman of the Elves, captured the mingled light of the trees in his Silmarils, the greatest work of craftsmanship by the Elves. After Beren recovers one of the Silmarils and delivers it to Thingol, Thingol takes the Silmaril and has it set inside the Nauglamir, the Necklace of the Dwarves, the greatest work of Dwarven craftsmanship. So we have the greatest of the works of the Ainur encased in the greatest of the works of the Elves, encased in the greatest of the works of the Dwarves. And it is possibly even more beautiful when worn by Luthien, the daughter of an Elf and an Ainu, married to a Man. And when that exact Silmaril is delivered to Valinor which sets off a chain reaction that ends with the final destruction of Morgoth, it is delivered by Earendil, the son of Tuor and Idril, and his wife Elwing, the granddaughter of Beren and Luthien.