Love and business are not always a good mix.

An important part of picking a partner is for the two of you to be on the same page financially, but doing that to the extent that you share income streams is not necessarily a cheat code. For a start, if the venture fails, then you are both dealing with the fallout of it at the same time, and on top of that, it gives you yet another topic to potentially argue over. 

At the same time, the opportunity to pursue a passion project together that may well turn a profit is the kind of thing that some people really enjoy. They see the payoff as worth any potential risk to the relationship. The prospect of a bonding experience that involves building something together is just too tempting to pass up. In fairness, trying to succeed in something like this is one of the ultimate tests of a bond between one person and another. It can either show that they pass the test of working together, or that one person is more invested in a way where things might end in tears. The only way to find out is to try.