Well, you owe money for everything you made above and beyond your initial investment usually. Example: I put $100 into Apple. Apple stock doubles and I take the money out (now $200). Even if I invest this money into another stock, I still owe taxes on $100 because that is money I haven't paid taxes on yet but it is essentially income for me (even if I reinvested it). Now if the Apple stock didn't do anything (stayed at $100) and took it out, I wouldn't owe anything because I have already paid taxes on the money before I invested in the stock.