Backpacking Europe Solo

I am a 25 yo dude planning a trip to backpack around Europe on March solo. I plan on going for about 3 weeks and visiting Madrid, Barcelona, Paris, London and Berlin. I would love to meet other dudes to hang out with. Do u have some tips for this trip? Did anyone of you have some advices or experiences about backpacking alone in Europe? I have searched a little and just can't imagine being there alone traveling for 3 weeks.