Why do some smart people find it difficult to keep their commitments?

Possible reasons are

1. Overthinking and over evaluation
2. Easy access to wide variety of options
3. Over critical and too wider imagination

Intelligence has a weak loophole. It presents you with too many options. Intelligent people are known to worry more than required as they are able to think of a bigger spectrum of thoughts. They need constant reassurance else their mind makes their life quite boggle some.