Not quite about selling but not buying them in the past is regret we all have that we have discussed on forum many times.But I would say it's not about bitcoin particularly any other investment that we have missed or sold at less price turns out to be a big thing in future gives us a feeling of regret that we should have hold it more.But for me I have sold my bitcoins for profits and when I was in need of money but still hold some stash.So not too much of those bad feelings.