Is it licking the sofa or its paws? If it's the paws, it's allergies. Often switching to a high quality, low ingredient dog food for sensitive skin and/or digestion will improve the licking.
It could be boredom too, which you can train her to stop doing. If you switch dog food and it continues,
this is a great thread about this where my suggestion worked. The OP resisted thinking it was silly and would never work, but she ultimately tried it and was amazed.
If he is licking the sofa itself, he could have an upset stomach.
Read this thread for info about that.