Two such opposite people shouldn't get along as well as you two do! But you do get along. In fact, each of you derives a lot of benefits from the other's way of doing things.
If there's any tension generated by your differences, it's most likely a creative tension that actually furthers your purposes. Your partner brings their spontaneous, high-flying creativity to the table and you bring your sensible, levelheaded approach to any problem, and then you two blend your strengths into one. Your partner is actually a really good influence on you in this regard; you tend to get a little too worked up at times about security and making plans, but they're able to jostle you out of this rigidity with their spontaneity and enthusiasm for life. They show you that it's okay to take risks from time to time. In return, you ground their sometimes off-the-wall ideas. You help them to see what's a good, calculated risk and what's just way too risky.