Trump suddenly fired the Director of the FBI. He led the investigation concerning relations with the Russian Trump. Maybe he gets too close to the answer? Nixon once did, but it did not save him from impeachment. What is your opinion?
There can be another reason. The director believed in the connection too much and would create suspicions when there should be none.
Think of it like this: you are an honest person and an employee, but a private eye gets hired to check if you're not a pedophile. He goes around the neighbourhood saying that he's investigating you because there's a suspicion you could be a pedo, but he'd openly say there's no proof yet and it's just a case he got paid to look at. I bet everyone would think you're guilty and wouldn't speak to you. He could get annoyed by those false rumors even if he's not guilty, but I agree that firing the investigator is a bad move. It will only give ground to more speculation.
In Congress are already talking about how to assign this investigation to a special Prosecutor. I think Trump is overestimating their capabilities. He thought that after the attack on Syria, he's rehabilitated. He no more suspected.