As a software product manager, we have to be cheer leaders for our team. We have to make sure our sales team, marketers, developers, the QA team are all staying pumped up about the products that we have asked them to sell/market/develop/test. But optimism that is not well grounded in reality is bound to backfire.
How many times have you met an optimistic sales person? They are always optimistic about meeting their sales quota – “Oh, I will make my $100K!” – when they very well know that the average deal size is $10K and there are only 3 potential deals cooking in the pipeline. How many times have you heard developers being very optimistic of shipping on time even though they are currently two weeks late and the original ship date is only a week away? It is also common for many of us to overpromise on what we can do for others, just because we hate to say No to someone.
To me, such optimism is very common but when reality strikes, such optimism does nothing but burn trust. Be optimistic about future success, but make sure it is grounded in reality. There are only 24 hours in a day and you are not inventing time to make the impossible come true, just because you are optimistic.
Trust once lost is hard to gain. Push yourselves to do more, but don’t set yourselves up for a definite failure. Under promising and over delivering is always a good option!
Software Product Manager by Gopal Shenoy 
So true. It seems there are those that are half full and half empty. It is always a balance.