You can't care about the problem more than the customer, or you'll go crazy.
That is not to say you shouldn't be proud of the workmanship of what you build (not quite the same thing as being proud of the product).
Unless you have a position where you have design authority, stop worrying about the why, and focus on the workmanship. Impress those that do have this authority with how well you do with what you're given.
If you believe that you have insights into making a better end product, then learn to communicate those insights at the appropriate time (before they've made up their mind). Try to get ahead of the curve and propose your ideas.
I'd agree that design and building are separate tasks and they can easily become too complex for one person, so it's best if it's divided. As long as someone 'owns' the design (for good and bad), it's less frustrating as a builder.
It sounds like in OPs case, vague specs means not good ownership of the design.
That is not to say you shouldn't be proud of the workmanship of what you build (not quite the same thing as being proud of the product).
Unless you have a position where you have design authority, stop worrying about the why, and focus on the workmanship. Impress those that do have this authority with how well you do with what you're given.
If you believe that you have insights into making a better end product, then learn to communicate those insights at the appropriate time (before they've made up their mind). Try to get ahead of the curve and propose your ideas.