Praise be to Allah.
Firstly: It should be known that Allaah, may He be exalted and glorified, is Wise in His commands and prohibitions. He does not command anything except that which is in peoples best interests and He does not forbid anything except that which is harmful. People may or may not come to understand the reasons behind these commands and prohibitions, although Allaah has His Wisdom in any given issue. Having said that, it is still not right to make following a command or prohibition dependent upon knowing the wisdom behind it; rather we should hasten to carry out the commands of shareeah, regardless of whether the wisdom behind it is clear to us or not. If it is clear, then praise be to Allaah, and if it is not, then the Muslim should not let the fact that he does not know it prevent him from acting in accordance with the ruling of shareeah. This is true submission to the command of Allaah, and Islam means submission to Allaah, may He be exalted, and obeying Him. If a person makes his actions dependent on understanding matters which may or may not convince him, he is in effect following his own thoughts and desires, not his Lord and Master.
The above does not mean that it is not important to look for the wisdom behind the rules of shareeah, or that doing so is forbidden, but we should not make knowing the reasons a condition for acting upon the ruling.
Secondly: there is no connection between things that are prohibited in this world and those that are allowed in the Hereafter. It is not correct to think of the Hereafter as being like this world; each realm has its own rules and laws. It is not correct to suggest that something is permitted in this world on the basis that it is permitted in Paradise or in the Hereafter, otherwise we would end up allowing things that we know from the basics of Islam are forbidden, such as drinking wine, taking more than four wives, singing etc. On this basis, the confusion reflected in the question about silk being forbidden in this world although it is permitted in Paradise should be cleared up.
Thirdly: The scholars have discussed the reasons for silk being forbidden for men in this world. For example, al-Allaamah Ibn al-Qayyim suggested in his brilliant work Zaad al-Maaad that
Among those who believe that there is reason and wisdom (behind the rulings of Islam) and they are the majority are some who answer that Islam has forbidden it so that people will keep away from it and give it up for the sake of Allaah, so they will be rewarded for that. Others reply that it was basically created for women, as is the case with gold jewellery, so it was forbidden for men because it can corrupt them by making them resemble women. Some of them said that it was forbidden because of what it may lead to in the way of pride, showing off and self-admiration. Some of them said that it was forbidden because when it touches the body, it makes a man effeminate and goes against his masculinity and manliness, so if he wears it his heart gains the characteristics of femininity and softness, as is obvious, even if he was the most masculine and chivalrous of men. There is no doubt that wearing silk will diminish these manly characteristics, if not take them away altogether. Whoever is too dense to understand this should just submit to the Wise Lawmaker.
(Zaad al-Maaad, 4/80). And Allaah knows best.