The answer to this question is subjective. Fantastic discussions can be created about what Dickens is trying to show about love and marriage through the various characters and their relationships in this book. To make things more complicated, Dickens was in the process of leaving his wife for a much younger woman; therefore, it has to be considered that he was letting his own relationships influence his writing.

A reader might be able to claim that Dickens is trying to show readers that marrying someone for purely financial reasons and/or the likelihood of statistical success is ludicrous. He shows readers this through Louisa and Bounderby; however, Stephen's marriage to his alcoholic wife is also shown to be a miserable agreement. The novel could be saying that marrying for love is a bad idea while at the same time saying that a marriage without love is an equally bad idea. A balance is needed.

The novel could also be saying something about love and divorce. Both Louisa and Stephen are falling in love with other people, yet they are not allowed to pursue those relationships because divorce is essentially impossible. It is possible to think that Dickens might be encouraging readers to always follow the path of love. When love runs out, get a divorce and move on to the next loving romance. Knowing that he was doing this with his personal life lends support to this notion.

I tend to think that Dickens' message about love is that the ability to love and to feel love is an essential part of human identity.  For individuals to strive to a realm in which this is not evident is an exercise in futility.  Through his characterizations and his direct message, Dickens seems to be suggesting that human beings are emotional creatures.  They are organisms that need love and need to understand its presence in their lives.  Certainly, Thomas recognizes this in his own failings as both teacher and father.  Louisa indicts him as much and Tom's own moral shortcomings demonstrate this.  Dickens seems to use Thomas as the embodiment and failure of a life that cannot understand the full implications of the emotional element in one's being.  The message here is to embrace the lack of a quantifiable element that is within love.  Through this, one can find the redemption and sense of belonging that a life dedicated solely to Rationalism lacks.  For Dickens, it is this notion of love and the ability to love that defines one's own being in the world.  It is the only element where some semblance of coherence can be found in a world where Rationalism points to greater fragmentation.