I have about 12 years solo travel experience and lonliness IS a problem. My rules for countering it as the following...
1. Stay in hostels, they are THE BEST place to meet other travelers.
2. If there are no hostels, go on 1 or 2 day activity outings, e.g. diving, climbing, canyoning, etc. They are another great way to meet people.
3. Carry your smartphone with you and join websites like Travlbuddy, Wandermates.com and VivTrav to help meet other people (like me http://www.wandermates.com/profileview.php?profileuserid=6 :) None of them are perfect, but they are useful.
4. Go on a backpacker bus, e.g. Paddywagon or Backpacker tours. They are brilliant also and a great way to hook up.