Voltage sources are to be added in series and current sources in parallel. Ideally voltage sources should have zero source resistance and current sources, infinite source resistance. Practical voltage sources have nonzero but low internal resistance. So when they are added in series, the source resistances sum up. Practical current sources may have high internal resistance and when a 100mA current source (with source resistance Rs1, say 100k) is added in parallel to another 50mA current source (with source resistance Rs2, say 200k), one can get 150mA. The source resistance of this 150mA current source will be Rs1||Rs2, (or about 66.7k)