In evangelical churches, it is often the case that men subjectively "feel" a call to become ministers, and they simply appoint themselves to be ministers of God. This was not God's pattern as described in Exodus.
And take thou unto thee Aaron thy brother, and his sons with him, from among the children of Israel, that he may minister unto me in the priest's office, even Aaron, Nadab and Abihu, Eleazar and Ithamar, Aaron's sons.
There are several points to remember about this passage:
1) God was speaking
2) Through another person in spiritual authority
3) He named a specific candidate
4) From among the children of Israel (i.e. the church)
5) He indicated a specific office
In Hebrews 5:4, we are told that ministers do not simply call themselves to ministry, but that any calling must follow the pattern given in Exodus 28:1:
And no man taketh this honour unto himself, but he that is called of God, as was Aaron.