1) "That's all that every happens here" is demonstrably false. While people tend to focus on the negative (a psychological issue not limited to political discussion and certainly not to liberals), there are many threads in this forum that have people espousing positive viewpoints, and some threads even dedicated completely to that. It probably is true that there are more negative posts right now than average, but most likely that is because there is a lot of horrible stuff going on right now. Whether you think it is the fault of the president or not, with more than 170,000 deaths from a historic pandemic, widespread social unrest, and the worst economic downturn since the Great Depression all happening within the last 6 months, it is hard to argue that this isn't the worst period for our country in decades. Maybe since the Civil War.
2) To me it is a very odd position to look at criticism as demonstrating a lack of love. When I see people who criticize their children more than I think is warranted, I typically don't doubt their love for the child. I may disagree that their actions are effective, but I assume that they are still trying to do what is best for their child.
3) I would think that one of the greatest signs of ACTUALLY loving America and not wrapping oneself in faux patriotism is to put country above party and/or personal interests. I see millions of everyday Americans who stay home not because they fear for themselves, but because they believe it is the right thing to do for their community, doing this. I see a number of public servants, from government employees to health care workers doing this. I see some ex-Republicans, the people associated with the Lincoln Group for instance, doing this. While I don't think it is the primary motivation of many Democratic politicians, I do even see some of them doing it. I do not see that being the case of any current Republican politicians (with maybe one or two exceptions) or of anyone who pushes false information about the pandemic.
4) Most importantly, the reason America is great is because of our core values that the country was founded on. The reason America should be loved is because it is has a foundation rooted in classical liberalism. It is great because it was created as a nation of representative government, equality between people, intellectual curiosity, etc. It has very often failed, sometimes in tragic fashion, to fully embrace and implement those core values, but the overall course of its 240+ year history is a movement toward a more perfect union through the promotion of those values. That is why me and many other people love the United States. America's greatness does not come from the Republican Party or the Democratic party, it does not come from any particular beliefs that either of those parties holds, it does not come a really high stock market, it does not come from using individualism or freedom (which on their own are good things) as a lame excuse to ignore science, and it certainly does not come from people like Donald Trump. I think that many of the people who are speaking out in the strongest terms right now understand that and it is out of love that they are so critical right now.