、。
When you do something yourself, it naturally becomes dear to you.
Literal
Anything self [with-で] undertake-[when-と], endearing becomes [general-truth-ものです].
A reflective observation using the generic truth pattern ~ものです ('it's the nature of things that / it naturally happens that'). なんでも ('anything / whatever') sets up the universal scope. 手がける ('to handle personally / to undertake') implies personal craftsmanship. いとおしくなる ('become endearing / grow dear') is an い-adjective (いとおしい) in its adverbial く form plus なる. The と conditional gives it a universal-truth reading: whenever X, always Y.