Here's why:
* The final scene: Ed and Winry are seen talking on a hill, overlooking the rebuilding of Amestris. It's a very peaceful and hopeful moment.
* Their conversation: They talk about the future, both of them expressing their desire to live normal lives and contribute to a peaceful world. Ed even says he wants to "make it a better place for [Winry] to live."
* Their emotional connection: The entire series has shown a deep connection between Ed and Winry, a bond built on mutual support, understanding, and shared experiences. Their relationship is full of affection and respect.
While a formal proposal isn't depicted, the ending of the anime leaves no doubt about their future as a couple. It's a happy and fulfilling ending, consistent with their journey throughout the series.
However, the manga offers a slightly different conclusion. Ed and Winry do share a heartwarming moment where they express their love for each other, but there's no direct mention of marriage or proposal.
Ultimately, the way you interpret their ending depends on your personal interpretation. However, the strong emotional connection and shared future aspirations between Ed and Winry leave a strong impression that they are destined to be together.