Underlying Asset Performance: The primary driver is the performance of the underlying stocks in the S&P 500. A rising S&P 500 generally pushes the ETF price up, and vice versa.

Supply and Demand: High demand for the ETF relative to its supply can increase its price, while increased selling pressure or creation unit activity can decrease it.

Expense Ratio and Fees: Although passively managed, the ETF's expense ratio and any trading fees affect the total return an investor receives, thus influencing its perceived value.

Market Sentiment: Overall investor confidence and risk appetite affect demand for equities, including S&P 500 ETFs. Positive sentiment increases demand; negative sentiment decreases it.

Economic Conditions: Macroeconomic factors like interest rates, inflation, and GDP growth can influence the profitability of companies within the S&P 500, impacting the ETF's price.

Currency Fluctuations: Given it's a fund listed in a specific currency, exchange rate changes, particularly against the US dollar, influence the returns for investors using other currencies.