When you have a job/career, provided by someone else, you are viewed as likely to be more stable in regards to your monthly income. Typically your monthly income is fixed or round-about the same as every month, providing said stability. This is viewed as lower risk, because they can judge how likely they believe you are capable of repaying your debt. Yes, you may lose your job/are terminated, and they take this into account as well, usually by noting how long you have been employed at the current location, etc. Other factors contribute to your ability to be take on debt, such as if you own your home or rent, and how much your monthly payments are, any other existing large loans, etc.
When you are self employed, it is difficult to gauge the level of stability you may have. One month may be great, the other not so great. It's also difficult to prove on paper exactly what you are paid since you are paying yourself (nobody to vouch for you), and it could be temporarily artificially inflated with the intent of getting a loan, etc. Being self employed, they may view you as higher risk also due to your priorities (ie. your business). In the event you fall on hard times, will you pay the debt back first and foremost or will you struggle to save your business (because it's the only method of income). This contributes, along with other factors, to you being viewed as a higher risk.
I'd wager different types of loans will be more accessible than others, such as a business loan if you are in fact incorporated or an LLC (I don't know about sole proprietorships but my guess would be it varies by state/country). The purpose of the loan will greatly effect you ability to take on the debt, ie. a personal loan because you just want some new things is not likely to be granted unless you put of collateral.
Of course, a lot of this depends on the lender and their policies/terms. I'm not an expert, so if this is a problem for you currently I'd recommend reaching out to a few different lenders and asking them to explain their polices.
