Entrepreneurship is highly appealing, and yet the reality of it isn't all that sexy -- especially for those in the early stages of building a new venture. 

Entrepreneurs face long hours. They miss soccer games on Saturday and family dinners on Sunday. Entrepreneurs miss many of the things that make other people's lives normal. And while others slave away within the confines of the standard 9 to 5, founders are working around the clock, working twice as many hours as the employed -- Elon Musk reportedly works 80+ hours per week -- something those outside the startup world often don't understand. After all, to many of those around you, you just work for yourself, right? 

But it's not just those looking at the empty seat at the dinner table that don't understand what it takes to build a successful business -- it's founders too. And this is a key indicator of whether or not an entrepreneur has what it takes to persevere and succeed. 

No matter how much we hear how lonely it is at the top, the reality is that very few actually imagine it to be. Instead, pop culture has glamorized entrepreneurship leading many to grand visions of champagne parties, lavish vacations, and a large, supportive group of friends and family there at your side -- anything and everything but lonely. But perhaps, for many, it's also anything and everything but reality. 

In the words of Shark Tank's most aggressive shark, Kevin O'Leary (a.k.a. Mr. Wonderful), "You have to understand you're going into a huge lifestyle change. Don't do it if you don't have the stamina to get kicked around and driven into the ground and get up and do it again." 

Part of the allure for many would-be or want-to-be entrepreneurs is, in fact, the lifestyle. But the vision of the lifestyle is anything but realistic. In many ways, it's like looking at a bodybuilder or swimwear model and saying you want to have that job so that you can have that figure. But you don't get that job before getting into that type of physical fitness, which takes a ton of work behind the scenes over a long period of time -- work that no one sees until it becomes a finished product. 

"This is no kumbaya stuff. This is hardcore stuff" --Kevin O'Leary 

It's not just recognition of the difficulty. What Kevin O'Leary's message reveals is that there's a vital aspect to reaching massive success, and that's learning how to be alone along the way. And it's not just a major indicator of a founder's ability to succeed, it's also an indicator that investors consider when making funding decisions.

In other words, is a founder comfortable being alone in the company of just their company?

This one question answers a number of questions. It reveals a founder's stamina, perseverance, drive, and devotion, among many other things that are crucial components of entrepreneurial success.

Keep in mind that being alone at the top doesn't mean founders at the top are lonely.

The crucial difference between being alone and being lonely makes the difference between founders who build successful startups and those who don't. Those who are able to get comfortable with the discomfort of flying solo are those who persevere and reach the summit.

To reveal whether or not a founder is truly comfortable with the reality of the often-grim lifestyle of an entrepreneur, O'Leary presents entrepreneurs with one simple question:

"Which is easier to replace: your business or your fiancé?"

Granted, this also doesn't mean founders must be alone in their personal lives just because they spend a lot of time on their own. But entrepreneurs inevitably need to be willing to make great sacrifices -- which sometimes come in the form of relationships with those who don't understand their journey. Luckily, for most, the answer is easy.

The good news is that even if you're not currently comfortable being alone, it's something that can be learned. The key is to learn it prior to building a startup, rather than as you build a startup. Getting comfortable with yourself enables you to get a solid understanding of yourself, and with that, your capabilities and limitations -- which also helps founders recognize when they need help. Not only does this help you build a better startup, it also helps you attract funding.

Investors want to know that a founding team is invested in their own business. And being invested means being devoted. Devoted means making sacrifices, and getting to the top and getting to where others can't means sacrificing what others won't.