There is no point in investing your heart and time and life experiences into someone who is leaving soon to only leave you broken-hearted when you COULD be sharing those experiences, time, and love with the person you are actually going to spend the rest of your life with. Every piece of your heart you give away to someone else is just one broken piece missing you can't give and share with your true love. If you expect something serious, discuss your intentions together. You have the right to ask them so you know what to expect and where your relationship is headed. If that makes them leave, it's less worry and decision-making for you and allows you to move on with your life. The longer you're in a relationship, the longer the healing process and the harder it is to move on if you end it.
No mater what happens, though, one bit of advice. NEVER SETTLE. Know your worth and don't settle for less than you deserve.