The end when Sebastian and Viola renite: "Were you a woman, as the rest goes even, I should my tears let fall upon your cheek and say 'Thrice welcome, drowned Viola!"
At the end of Trevor Nunn's film version, there's a shot of Viola and Orsino kissing next to Olivia and Sebastian — and Viola and Sebastian reach out to hold hands. You can feel the joy.
Many productions add a WAFF-y feel to Viola and Olivia's first interactions, including Olivia showing her face to "Cesario", despite previously refusing to, and Viola gently joking with Olivia and praising her beauty, and generally treating her like a person, which Orisno, although he meant no harm, was not doing. It's easy to understand why Olivia fell for "him."