If a bolt weakens over time, it happens by growing a fatigue crack. The tensile prestress (tightening the bolt) reduces the fatigue stress and prevents the growth of a fatigue crack. Bolts are designed to be prestressed by tightening so that the stress on the bolt is below the yield strength in the elastic range of the stress-strain curve. If the bolt is initially tightened to a lower stress level (lower foot-lbs of torque on the wrench), the fatigue cracks will grow faster.

If you want to understand this a little better, take a look at the Metal's Handbook - Edition on Failure analysis. There's a nice chapter explaining bolt and fastener design theory.