我之所以决定辞职是因为这份工作不但薪水低而且也接触不到太多的朋友。The salary keeps so low and furthermore I cannot get in touch with many new friends and consequently I decide to quit. 这句话中的“decide”用不用换成“decided”
回答你的问题,用完成时 have decided 较合适,因为你已经决定了。
但顺序上,应该先写你的决定,再写理由。
按你的意思可以翻译成:The reasons that have made me to reign are low pay and not much opportunity to meet more people. 或 The reasons why I have decided to resign are ...
请参考。