Ok, sorry this is going to be a bit rambling, but keen to know if anyone else has had a similar problem. To cut a long story only a little bit shorter - my Mum and Dad were divorced over 20 years ago - Mum remarried (then widowed) and has since had other relationships.
Dad has never had another relationship in all that time
He has suffered from depression on and off during that time (I think he also suffers from S.A.D, but never properly diagnosed). During my childhood my parents had there fair share of arguments, etc immediately after the divorce, but since then have always got on and been friendly towards each other (Dad and my Stepdad were even friendly after the initial awkardness) - and live just 5 minutes from eachother. Dad does odd jobs for my Mum as she now lives alone, has done her decorating, etc and she helped him to revamp his whole flat. He has always visited my Mum's house on family occasions such as birthdays, Christmas, etc, and had/has dinner there often (even when Stepdad was alive), if either me or my brother were there to visit too. Trouble is that its clear to everyone (he has even admitted it to me) that my Dad is still in love with my Mum, and hangs around more than Mum would like. They have a set of mutual friends (who tbh my Dad didn't see that often when my Stepdad was alive, as they socialised with my Mum and him more). Now my Dad is always being pally with them, but more annoyingly for my Mum, he always tries to be too friendly to her other friends and thinks he's 'in' with everyone.
He sort of depends on my Mum a lot, and my brother and his girlfriend, in terms of socialising, and hasn't got many friends of his own. It's not that he's shy - he's fine when he meets new people, and is very talkative and friendly - but shows no inclination of finding his own friends / girlfriend. In fact, he would be horrified to even think there is a problem here, and would get very upset if any of us suggested that he tries to widen his social group.
Also, he goes through certain times of the year (like now) when he gets overly generous and effusive, calling everyone all the time, generally getting over excited, etc - it sounds silly, but it's driving my Mum and brother mad (my brother gets a bit annoyed with me too, as I don't live near then all - about 3 hours away by train - and don't have to put up with it like they do).
REALLY sorry for such a long post - wouldn't be surprised if I don't get any replies to this!