Conservatives are generally seen as hard-asses (tough on crime, blow countries up, etc) and this stereotype his broadcast by the Right AND the Left.
But I believe that the fundamental causes of this phenomena is a more subtle, psychological response. For modern Liberals negativity is chic. For political reasons, they will often downplay good news and spotlight bad news. Everyday Liberal "conversations" have a negative tone because they are largely talking about grievances. Their worldview tends to be more pessimistic, defeatist, and cynical.
That doesn't always jive well with the basic notions many hold about the USA and life in general. Many people still hold onto the ideals of American exceptionalism and long for positive, optimistic messages. Reagan understood this and it's a reason some people warmed to Obama's Hope Train. Kennedy didn't talk about how crappy this country -and the people- were, he talked about how great this nation is... he understood that to make something better you build it up, not tear it down.
A lot of people intuitively realize that pessimism undermines our ability to solve important problems. Anger and despair drain people of life... and it drains a country of life. People want a strong America represented by people who talk with national self-esteem. Modern Liberal thinking, rife with pessimism, defeatism, and cynicism, saps our strength and determination, impairs our national health and weakens our ability. This is the root why the Left is often seen as weak.