1. Trump repeatedly downplayed the Russian threat before being elected
2. During an interview when Putin was deemed a murderer, Trump said, "you don't think we kill people too?"
3. Trump to this day, is still openly combative toward Sessions recusing himself and not protecting Trump. Says he would have hired someone else.
4. Trump fired Comey and says, "It was because of this Russia thing".
5. Virtually every one of Trump's associates visited, has ties to, or met with Russians.
6. Trump lied about his Russian hotel deal. Said he, "Had no business dealings, future business dealings and was making no plans for deals" when he was still making a deal on his Moscow hotel.
7. Trump believes Putin over American agencies who all say there was Russian involvement in our election, during that hopelessly pathetic news conference where he sucked his dick for the world to see.
These are just a few instances and reasons to investigate. The list is endless.
The straw that began the investigation was firing Comey.