Sheesh. Medicare, Social Security, education, public lands, immigration .... there are plenty of things that could lend to a positive message - without even having to dwell on what a mess the Trump Administration is making of all those things.
the problem, however, is this:
the people who are going to vote for the democrats (ie, democratic voters) are going to vote for democrats either way, so the question is just one of motivating them to bother to vote - as in, the deciding factor is whether or not they get up and get out and vote in sufficient numbers or if they don't bother to vote.
now, whether it's true or not, it's currently the opinion of the cultural zeitgeist in the US that fear and/or anger are better motivators to encourage voting than hope or positivity or self reflection - the former gets your base out in droves, the latter gets you fuck-all (see the 2016 election for evidence of that)
the message of the democratic party itself is inherently one of positivity and change, because democrats are the party of "generally working to increase the quality of life for all citizens of the country so that everyone is living by a standard of a 1st world country", which is clearly the polar opposite of what republicans are pushing for.
the problem is that all the people who should be voting democrat but instead vote against their own interests in every practical way are doing so for dogmatic ideological reasons, and are too pants-on-head retarded to ever be able to see how idiotic they are.
so emphasizing the positive aspects of the message "vote for us to protect livable wages, social safety nets, and utilizing our first world status to enhance the lives of everyone" is a wasted effort on right-wing thinking people because they don't value any of those things, they only value on increasing the net worth of rich white men and oppressing others under the guise of jesus.