Other comments relate the high income tax, but Spain also has a "impuesto de patrimonio" wealth tax, which forces you to declare all worldwide assets.

Even if the current rates are low, they may (will) rise. It is surveillance masquerading as taxation, until the rates go up and then your are trapped, gutted and left for dead.

https://www.spainmadesimple.com/finance/wealth-tax-in-spain/...