Why is the thioester bond weaker than a regular ester bond?

Sulfur is a much bigger atom than oxygen or carbon. As a result, resonance donation from the sulfur into the carbonyl carbon is limited, and the C-S bond doesn't have as much multiple-bond character as the carbonyl carbon-oxygen bond in an ester.

What are the contributing factors to the fact that thioesters are less stable than their oxygen counterparts?

Kinetically or thermodynamically?

Stable with respect to what?

If we're talking about thermodynamics, and stability with regard to hydrolysis, then yes, thioesters are generally less stable than esters. Complete hydrolysis of a thioester or an ester will return either to a carboxylic acid. So if we have a thioester and its corresponding ester, the stability of the product is the same.

The stabilities of the reactants differ. The thioester is higher in energy than the ester because of its weak C-S bond. It's more favorable to turn a thioester's C-S bond into a C-OH bond than it is to turn an ester's C-O bond into a C-OH bond.

Why is the thioester bond weaker than a regular ester bond?

Sulfur is a much bigger atom than oxygen or carbon. As a result, resonance donation from the sulfur into the carbonyl carbon is limited, and the C-S bond doesn't have as much multiple-bond character as the carbonyl carbon-oxygen bond in an ester.

What are the contributing factors to the fact that thioesters are less stable than their oxygen counterparts?

Kinetically or thermodynamically?

Stable with respect to what?

If we're talking about thermodynamics, and stability with regard to hydrolysis, then yes, thioesters are generally less stable than esters.

It is more favorable for water to attack a thioester's carbonyl carbon because it has a greater partial positive charge than a corresponding ester's. This is due to the resonance issue discussed above. A thioester also has a more stable leaving group, as the larger size of sulfur allows negative charge to be distributed better.