It is important that you have someone else there, preferably someone sober or at least more sober than you. You need a baseline to calibrate your sense of what is real and what isn't, so you don't get lost in your own head. This is very important.

When you are really experienced and no longer have any anxiety about what lurks inside your subconscious, only then is it advisable to trip alone.

No, the two implications are that 1. You should be somewhere you feel safe. Preferably with someone you feel safe. and 2. It is better to be outside in nature. (1) is always spoken of as being crucial and (2) is something that is oft repeated. I don't have any experience myself to say why (2) is so often suggested.

The whole being outside in nature thing is because of the character of the visual perceptual distortions that occur on psychedelics.

Basically it perturbs the brain's pattern recognition systems, and causes them to look much more strongly and insistently for patterns and regularities in the input. Therefore, to get the best effect, one should feed in highly irregular images, such as the messy, fractal-ish geometry of nature.

When you look at tree bark or grass or clouds on psilocybin, your brain tries furiously to see a pattern or regularity in it, and as a result your perception of it constantly shifts and rearranges.

When you stay indoors and look at manmade things with human-conceived geometric shapes, the patterns are already there and your brain sees them immediately. At best this is boring, and at worst it can cause you to become fixated in a way that is quite uncomfortable.

This isn't the most eloquently worded explanation but I'm sure most people here can tell what I'm talking about. Psychedelics have a lot to teach us about the workings of perceptual cognition as well as about more spiritual matters.