Both traveling alone and traveling with friends have its benefits and downfalls. It's up to you to decide which you prefer in certain scenarios. I personally prefer traveling either alone or with a small group. It's my personal opinion that big groups are too difficult to manage on big trips.
Here are some pros and cons to traveling solo vs. traveling in a group.
You can do whatever the hell you want, whenever you want to do it.
The best part about traveling solo is that you don't have to wait for anyone else. Perhaps this is moderately selfish. But traveling should be selfish. When you're in a group, you have to take into account what everyone else wants to see / do, everyone else's getting ready times in the morning, hunger/diet preferences, when people have to go to the bathroom, others monetary and budgetary concerns, dawdling around after meals etc etc. When you're by yourself, you can do things on your own time. If you want to sleep in, you can. If you want to spend 5 hours in an art museum, you can. If you want to spend $100 to ride a gondola, you can.
Ultimate Me Time
UMT is great sometimes because you get to be alone with yourself. You can read, catch up on knitting, read your emails, talk to yourself, and pretty much just be at peace with yourself. Everyone needs a little UMT and time out for themselves, so do it!
More Leeway to Meet Cool People
Being by yourself puts you in a position where you're forced to converse with other people. Imagine that! But when you're by yourself, people are usually more likely to come up and chat with you, as groups can be intimidating to solo travelers.
You have to rely completely on yourself
The coolest part about traveling alone is that you must rely entirely on your own instincts for everything. You don't have others to lean on to figure out how to use the metro, read maps, or communicate in a different language. Solitude is nourishing, and the benefits to this are innumerous.
Can be ~lonely~
Sometimes, solo travel can be lonely. There are ways to combat this.
See my article "Why Solo Traveling is Less Lonely Than It Seems."
Traveling alone can be scary because you may have concerns for your safety when you're traveling alone. It's important to be vigilant at all times, whether you're traveling alone or with others.
If you're a girl and thinking of traveling alone, see my article "Tips to Staying Safe as a Female Solo Traveler."
Makes seeing the nightlife more difficult
One of my personal rules is that I never drink more than one drink if I'm traveling by myself. Sometimes, this can make seeing the nightlife more difficult. If safety is a main concern, it can be difficult to see the nightlife to its full extent when you're traveling by yourself. Definitely don't shy away from going out to eat in a bar/restaurant in the later evening, though. It'll give you a glimpse of the nightlife, without putting your safety in jeopardy.
No one to take your photos :'(
Perfecting the selfie is no easy task.
You have to rely completely on yourself
This is a really positive thing! Most of the time. Until you get lost in the middle of seemingly nowhere and have to depend on yourself to get around. Relying entirely on your own instincts can be scary, especially if you're directionally challenged, worried about eating alone, or know none of the language. But it gets easier as you get more comfortable. Be comfortable in yourself, be confident, and relying totally on yourself won't be as much of a 'con' anymore.
Traveling in a Group
Obvs, banter is one of the top reasons that you may be better off in a group. You'll spend the whole time laughing, creating new memories and inside jokes, playing drinking games, and bonding over new experiences. Banter is way less fun when it's just inside your own head....
You may get to see things you otherwise wouldn't have known about
"My friend told me that I have to see....." Having other people around will open up your world to a whole realm of possibilities and new experiences. Friends know friends who know friends of friends who can suggest awesome activities and foods to try. You may have never even heard about them without them.
Seeing the NIGHTLIFE!
You're with a group! Hit da clubs!!! Seriously though.
Traveling with a group can lead to some awesome stories. The time you got super lost in the back roads of Paris and it was LOLZ, the time you couldn't speak Italian and you ended up using hand gestures and the store clerk thought you were making an inappropriate gesture at him, HAHAHA. All those things are way less funny when you're by yourself.
Sharing your bliss
Sharing your happiness and experiences with someone who understands is a huge benefit to traveling with others. Sometimes, when I'm traveling alone and I'm struck by something in particular (whether it be food, culture, architecture, a sunset, whatever), it would be nice to share it with someone. Having a partner by your side will allow you to share that bliss.
Taking everyone else's _________ into account.
Bank account? Taste preferences? Sleeping habits? Walking pace? You name it, suddenly you're responsible for it. Having to take everyone else's preferences into account will slow you down, without a doubt.
Minimal Alone Time
Everyone needs some Ultimate Me Time (UMT) on occasion. Especially during long trips, being with a group can be exhausting. There's little leeway for alone time, and few windows of opportunity where you can slip out on your own.
Conflicting Travel Priorities
Here for the party? Want to see the architecture? Planning on leaving the big city for the countryside? I recommend talking about these things with your group before you leave. A little mix of everyone's priorities is nice, but it's important to know everyone's expectations before you get there and it explodes in your face.
Personal preference is everything. Personally, I like to mix it up. I find that there are more personal benefits to solo travel. I love the way that it challenges me, makes me think, and makes me feel vulnerable. But traveling in a group or with a buddy is fun too, and has its advantages. Always traveling alone can get boring sometimes. If you don't feel comfortable traveling alone, don't! There's no shame in group travel whatsoever. Any and all travel is beneficial, so no matter who you go with, you're always in for an experience of a lifetime.