His sentimentalism led him to keep every old photograph, even though they had turned yellow.

sentimentalism 是由形容词 sentimental（多愁善感的、感伤的）加上名词后缀 -ism 构成的名词。sentimental 来自 sentiment（情感、情绪），-ism 表示"主义、特性"，所以 sentimentalism 指的是"多愁善感的性格"或"感伤主义"，在这里特指"怀旧情结"。