Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
Praying istikharah is recommended. It is not obligatory. Not praying istikharah does not mean that this matter that is going to happen is utterly evil. Once a Muslim reaches a decision, he/she should go ahead without hesitation. Hesitation is a result of the insinuations of the shaytan who would ruin people’s lives through doubts.
Allah Almighty knows best.