In order to be Muslim, we maintain a balance between fear and love in Allah aza wa jal.
A person cannot be Muslim unless he/she loves Allah more than anyone and anything. Otherwise they would be committing the shirk of love, meaning polytheism by loving someone more than Allah.
Same goes for fear, fearing anyone more than Allah is the shirk of fear and polytheism. We Muslims fear Allah due to the severe torment he warns mankind with in the Noble Quran, that mentioned in this life and in hell. We love Allah because of his mercy and because he promises those who believe and do good deeds and avoid the great sins, paradise, which no one can ever earn, but he gives it out of his mercy.
In Islam it is required to have hope in Allah's mercy, and to despair at his mercy, i.e. believing you will never be able to enter paradise or be forgiven, is kufr and a flawed belief.
Hopefully this answers your question. There's nothing wrong with someone becoming Muslim initially due to the fear in Allah, Allah tells us in the Quran that when we hear the name of Allah, a believer's heart feels a tremor out of fear of him, and then he calms down after remembering Allah's mercy.
The Prophet (s) used to cry during the night prayer out of fear in Allah, as have many other pious individuals in the history of Islam.