Enthalpy - Chemical Reaction?

It might help to think of this in terms of bond dissociation enthalpies. The reaction you posted is the opposite of atomizing Cl2 from a molecule to 2 Cl atoms. When you break a molecule into its constituent atoms, you put energy IN to do this. Think of it like breaking a stick; you have to provide energy to break it. Since 2Cl-->Cl2 is the opposite of this, energy is released instead. 2Cl has a higher enthalpy than Cl2 for this reason.