第5行的on the one hand 去掉the,不需要定冠词
第6行的colourful,您指的是多彩吧?可以把句子改成：On one hand,western festivals have very rich/varied contents,so they can make people feel happy.
这里的happiness是名词,应当为feel+形容词（happy）
第7行becomes改为becoming,poeple后面的部分也改为 are more willing to .以配合上文的递进关系
第9行的decreased改为has decreased.become去掉
最后一句重新整理为：
This phenomeno makes culture colourful while decreasing our intention of protecting troditianal (Chinese) festivals.