I would suggest that you either join the clauses with "because" or make it two separate sentences:
I just want the best for you because I have your best interest at heart.
I just want the best for you. I have your best interest at heart.
2022年7月24日
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I just want the best for you because I have your best interest at heart.
I have your best interest at heart and that is why I want the best for you.