Don’t get enamored with the complexities you have learned to live with (be they of your own making or imported). The lurking suspicion that something could be simplified is the world’s richest source of rewarding challenges.
Dijkstra warns against becoming so accustomed to complexity that it begins to seem necessary. A complicated system, theory or method may contain features inherited from earlier decisions rather than requirements of the problem itself. Familiarity can therefore make unnecessary difficulty harder to recognise.
His alternative is to treat the possibility of simplification as an intellectual opportunity. Asking whether something can be expressed, designed or understood more simply can reveal assumptions that have gone unexamined and lead to more fundamental solutions. In the context of advice to a young scientist, simplification becomes a productive direction for research rather than merely a matter of elegance.
The quotation also concerns habits of thought. Expertise can make people comfortable with complications they have learned to manage. Dijkstra encourages researchers to retain enough distance from established methods to question whether those complications are genuinely unavoidable.