Love
Love, like in many other works of literature, is a theme of the poems of James Russell Lowell. Especially in his poem entitled "Love", Lowell explains that love is not something that needs to be complicated. It is, in fact, the simple beauty of sitting together with friends and family that is True Love. There is no need to complicate it with relationships and mischief, among other things - one must simply acknowledge that the presence of others is what makes life fulfilling.
Freedom
In a time when slavery was still prevalent and many countries were undergoing restructuring because of peasant revolutions, freedom was a very relevant theme. Many of his poems tell of freedom symbolically, but overall, they give a theme of universal freedoms. In one of his poems that does not show freedom as a symbolic element, even slaves are mentioned, which is relatively unheard of for the time. It is because of Lowell's explanation of universal freedoms that apply to black and white, rich and poor, that his poems have become so popular.
Better Conditions
In a way, this goes along with the theme of freedom, but better conditions in life and true freedom are completely different things. Lowell was a strong advocate for better rights for factory workers, and also was against capital punishment. These beliefs bled into his writing, and eventually influenced the minds of many readers. In an era where children were forced to work sixteen hour days in polluted environments, it is not hard to understand why Romanticist writers like Lowell developed a sort of cult following for their "rebellious" beliefs.