A lot of grand and lofty plans are made in Middlemarch (see the "Dreams, Hopes, Plans" theme), but not a lot of them get carried through. In fact, most dreams in this novel are crushed by the mundane but necessary pressures of everyday life. Dorothea can't carry out her plans because she's a woman, or because the projects are too financially risky, or because she's told they aren't realistic. Lydgate doesn't finish his life's ambition (to make a major medical discovery) because he's too busy dealing with financial problems and debt. Casaubon never finishes his life's work because he's too caught up in the gory details. Most of the dissatisfaction in this novel is a result of a failure to reconcile lofty ambitions with everyday realities.
Celia and Mary Garth are the only two major characters in Middlemarch who don't experience dissatisfaction, because they are capable of reconciling their hopes and desires with realistic possibilities.
In Middlemarch, George Eliot shows that dissatisfaction is an inevitable result of the too-rapid progress of modern life.