I haven't been an investor for very long, but even in that short time there's one mistake I've seen more than a few founders make, and that's to fall in love with the product they're building. I know what you're thinking: how can that possibly be a bad thing? Don't investors want people who love what they're working on? Well, not exactly. You want to back founders who are passionate about their products -- in fact, it's odd when they aren't -- but there's a critical difference between being passionate about what you're building and being in love with it.
That may seem like a narrow gap, but it all comes down to motivation. What investors want is someone whose animating purpose is to solve a particular problem (especially one that people don't realize yet that they even have!) or fix something they think is broken. The product is just a means to that end, not an end in itself. To put it another way, you want someone who is more in love with fixing a problem than with the product itself.
What often happens is that a founder puts so much time, effort, and/or money into what they've built that they become blind to its shortcomings. Or, you'll see a founder become so enchanted by how clever or original they believe their product is that they don't want to listen to feedback they're getting from the market that there's no demand for it. This has nothing to do with the effort put into creating it or even how "good" it is -- I'm not talking about situations where the product itself is shoddy. Almost every product that's pitched to me is well-made. Quality is rarely the issue. What's wrong is that these are products which excel in every area except the one that matters most: being something that people want to use.
It doesn't help that the message a lot of founders hear when they encounter a roadblock like this is that they need to ignore the haters and just keep pushing and pushing until they make it, as if sheer perseverance alone can turn a failing product into a successful one. They have such a strong vision of what the product is supposed to be that they're not willing to change course and make significant changes to it. They'll convince themselves that the problem isn't with what they've built, it's that the rest of the world doesn't get it yet -- or worse, they haven't gotten the right people (i.e. bloggers or "influencers" on Twitter or Product Hunt, etc) to pay attention to it. So, they keep repositioning the product and what it's for, trying to find different problems for it to solve.
That's why you want someone who is in love with a problem, who cares more about solving that problem than anything else. A founder like that fixates on that problem and then keeps trying over and over until they figure it out. Part of that means being willing to make whatever changes are needed to the product to make it work, up to and including jettisoning what they've already built and starting over with something entirely new.
It's not always easy to tell the difference between being in love with a product and being in love with a problem, especially when you're in the middle of building a company. There will be days when you don't care at all about solving the problem you set out to solve. Other times you stumble onto something which solves a problem you weren't even aware of, but it is one you end up caring deeply about anyway. In the early days it can be difficult to know whether your product is working or not, and it's natural to want to give it time in the market. But unless you're lucky enough to have one of those products which connects right away and just takes off, sooner or later you have to confront where you're at and figure out whether you're building something which solves the problem you're tackling. When those hard days come, confronting the shortcomings in your product is a lot easier when your overriding drive is finding a solution to that problem.