Stelliums and interceptions are both imbalances from opposite ends in the natal chart. With a stellium, there is an excessive amount of energy in one sign or house in the chart. With an interception, there’s a lack of energy in two signs and houses in the chart. Either way, the native attempts to invest a lot of energy in these areas. With both areas, the native strongly tries to control these areas for different reasons.
For a stellium, the reason why so much energy is invested into it is because with many planets, they are many potential ways for the native to express themselves. This is especially true if the Sun and/or Moon is part of the stellium. There’s also a struggle of controlling the stellium, since there’s so much energy. When a native struggles to control a stellium, it can often feel like tightly holding a bunch of sand. Even if you try very hard, your attempts to control where the sand goes is futile.
For an interception, the reason why so much energy is invested into it is because with a house, sign, and maybe planets locked away in the natal chart, the interception can often feel like a black hole. Since there’s a sense of distance and lack of access with an interception, the native can attempt to invest in the interception to close the distance. With that being said, the native can often try their hardest to access their interceptions, only to see their energetic efforts falling into a distant void.