Well, it could all come down to Lister finally FINALLY giving up hope for ever making a life with Kochanski. Dave's not "stupid" and nobody knows better than me about carrying a torch for someone who's gone, but eventually hope dies and is replaced by resignation. I mean......... how long has it been since she's even been slightly interested in Dave? The male ego can only take so much rejection and Dave wouldn't be immune to that. Long nights alone except for alcohol does lead to soul searching and sometimes bitter truths about yourself and those around you.