In legends and cultures in general, people are completed and happy only when they have family, because it defines continuity of a person, or shows that struggles may be overcomed by having someone beside you. That is the reason why the grand prize for achieving great deed in legends usually includes getting a girl/women for her to become heroes wife. In other times wife is the biggest sacrifice, or the reason for great things to happen.
Legend about establishment of Trakai Castle is a story of how Birute, wife of the grand duke of Lithunaia Kęstutis, wished to be be reminded of her home of which she was missing a lot. Birute came from the shores of the sea, so she missed water and waves. Duke loved his wife and wished her to be happy, this is where the decision to build a castle in the island of a lake came. This is how one of the old capitals of lithunaia – Trakai was established.