Praise be to Allah, and may peace and blessings be upon the Messenger of Allah. To proceed:
There is no contradiction between believing in the decree of Allah Almighty and affirming that a person possesses will and choice. Allah has affirmed that human beings have a will and a choice. He says:
“For whoever among you wills to take a straight path.”
At the same time, He, Glorified and Exalted is He, says:
“And you do not will unless Allah wills.”
Accordingly, a person may choose whom he wishes to marry and strive to achieve that choice. However, his choice and will are themselves within the will and decree of Allah, and everything that occurs is written with Allah Almighty.
A person may strive for something that he wants, and then obstacles may come between him and it. In such a case, he should know that Allah Almighty did not decree for it to happen for a wisdom known to Him. A person does not know the unseen or the consequences of matters. He may dislike something in which there is goodness for him, or love something in which there is harm for him. Allah Almighty says:
“Perhaps you dislike something while it is good for you, and perhaps you love something while it is bad for you. Allah knows, and you do not know.” [Al-Baqarah: 216]
Therefore, the conclusion is: A person chooses his life partner through his own will and strives to attain that choice, but this very choice is part of Allah’s decree and will. Whatever ultimately occurs is predetermined and written by Allah Almighty.
And Allah Almighty knows best.