You might ask why not, as my partner is the person I trust most. You are (hopefully) right when it comes to trust. But when you start a business with you private partner the business becomes your life. That means 24/7. You will work together, talk in your free time about work and the business starts taking over not only your business life, but also your private life. And all what a happy partnership can give and take will be eaten up by the business. Sounds negative, I know.
I started a business with my wife, now ex-wife. We become entrepreneurs with our start up and it was a exciting time. And we enjoyed being self emplyed and to life the dream of creating a business. And as good as we were as a couple and team in business we did not recognized that we lived for the business and the business took over our lifes in a way that is not good for young couples. After 3,5 years we got divorced. We are still good friends, but due to the fact that we spend more time together in 3,5 years than other couples in 10 years (with seperate jobs) the relationship simply died.