As fun as it is to travel with friends and family, there’s nothing better than the freedom of doing what you want, when you want without having to wait on anybody. I've been doing it for two years now and can safely say it's become an important part of my life.
But solo travel isn't everyone's cup of tea and it may be due to the following:

You crave attention

If you're not comfortable with being alone, then solo travel is not for you. Whilst you will meet other people on your travels, you'll most likely spend most of it alone, meaning there’s no-one to shower you with compliments, to listen to your every word or even talk to at all.
NOTE: if you look foreign to the country you’re travelling to, then chances are you will get lots of stares, but that's not the good kind of attention

You’re not willing to step out of your comfort zone

When I told people I was heading to Sri Lanka for my fifth solo trip, they all said  ’ OMG, YOU’RE SO BRAVE.’
I guess a woman — a black one at that — ­­travelling to a different continent alone is quite brave, but I don't do it because I see it as a courageous act. I do it because I like exploring new places and experiencing a country’s culture, the food, the people. The world is much bigger than the continent in which I reside.

If you're not willing to embrace it and to try something new, then don't go.

You’re worried about what people will say

‘Don't you have anyone to go with?’
‘Won’t you be bored?’ 
‘Why are you going alone?’ 

These are just some of the questions you’ll get as a solo traveller because those who aren't into solo travel don't understand its appeal. You may also get these questions - often with a strange look or two -  from the locals. Eating dinner alone is my least favourite part of solo travel because of this, but a girl needs to eat so I just deal with it.  
P.S. as a woman, you will get asked where your husband is, regardless of your marital status.
You believe everyone should speak English

Oh, ignorant Westerner, why do you expect someone who lives thousands of miles away from you to speak the same language as you? Granted, English is one of the most spoken languages worldwide, but that doesn't mean everyone SHOULD speak it. Learning a few of the local phrases such as 'hello' and 'thank you' won't do you any harm.

You think travel is expensive

Obviously, if you want a 5-star luxury resort, then it will cost you some coins, but travel doesn’t have to be expensive and sacrificing some of the small things like that daily flat white can add up to a decent travel fund. Humble yourself, DIY your beauty treatments at home and live your best hostel life.
Travelling around Europe (for now, pre-Rona) is pretty cheap, but travel doesn’t always have to be in another country; living in my London bubble, I tend to ignore the fact that beaches and quaint villages exist in the UK. 

Your planning skills leave a lot to be desired

Booking flights and accommodation, getting to and from said accommodation, planning your itinerary, finding the right transport, managing your budget - some of the key components of solo travel. All which you have to do by yourself because Google can't do it all for you.
If you don’t know your Maps.Me from your Google Maps, then get yo' life together.
I like being organised and find it much easier to work towards a plan; it doesn’t have to be set in stone as I like to keep it flexible, but having a rough idea is important.
Despite all that, I still believe everyone should travel solo at least once in their life, but there are some things you need to consider before embarking on your trip. The thought of being in another country alone might turn your stomach, but as with all things in life, it'll get better with experience and you might actually have a good time. 

Or it just might not be for you. Who knows?