When is tax painful?
Tax need to be a willing obligation and any taxpayer should not hesitate to pay. This is because without tax no government and without government no peace, no fair sharing of social and natural resources, etc. Tax becomes painful when it is not drawn for social benefits and needs. It is also painful when taxpayers are not treated equally.
It is true that rich countries governments donate and subsidize poor countries from funds contributed by taxpayers. This gives value to domestic taxpayers as humans helping their brothers and sisters and tax cannot be painful as long as the donating government is open to its society and the receiving government uses it to the purpose intended and agreed.
In short, tax is painful when taxpayers are not treated equally, when tax is not used for social benefits, when governments are not open to their societies and when the tax collector is felt dictator over the taxpayers.