This may sound weird but I love going to concerts alone.
If you go with friends and they don't love the artist as much as you, you feel this awkward responsibility or obligation to curb your enthusiasm. This happens, and you're standing around trying to explain and make excuses as to why the artist isn't connecting with them, basically bending over, ass up, trying to justify why THEY aren't enjoy it and killing your enjoyment because your friend is emotionally raping you over a $20 ticket. That emotional burden ain't worth the company.
If you go with a girl, and the concert is turnt up (which it always is if I am there), then you have to either protect her or stand in the back, or grind on her to some shit that isn't fitting the mood. This is the worst situation because it's either pussy or going HAM for the artist you've been waiting to see. Having had this happen to me when Cudi came to my school last year, and my ex having no idea who the fuck he was, I couldn't even get a little turnt up. Not even a little bit. What a goddamn shame that was for me (not really it was only Cudi) but awkwardly dancing and grinding with her drunkenly to Cudi for two hours was probably the worst way that concert could've gone.
Or you go with friends and end up getting lost from them because you're getting ratchet, and now you feel obligated to find them and reconnect temporarily between songs. This has happened to me, and is the BEST situation you can have with friends at concerts.
But if you go by yourself, what usually happens?
You go in mad awkward, you smoke or drink, you go HAM, you USUALLY end up bonding with the people around you who all want to be there as much as you for the same reasons, and you have fun.
I went to a shit ton of concerts the last couple of months, with friends of all varieties and by myself, and the best experience I had was going to a concert where I only knew one band on the bill and I was by myself.