Looking at the many concerts I missed because of that, I decided to automatically buy 2 tickets for each event and ask my friends afterwards, stating that I had an extra ticket. I ended up attending many concerts with one of my friends, and rarely had to sell my extra ticket.
On a side note, two years ago, a group of friends invited me to join them for a 10-days trip in Turkey. They would fly directly from Paris to Istanbul. I lived in Bordeaux then and figured: why don't I go to Istanbul by train on my own? It took me 2 weeks to get there, and along the road I stopped at Milan, Florence, Venice, Ljubljana, Zagreb and Belgrade (plus Munich, Vienna, Budapest, Krakow and Berlin on my way back).
I knew that noone would have agreed to come with me and I immediately felt that I had taken the right decision. It was a sudden one but I don't regret it. I met wonderful people and visited beautiful places, and I realize that depending on someone else's decision would have prevented me from doing these things.