Why did Theodore Roosevelt have to proceed with caution on pushing for reforms in his first term?Isn't it because the congress was on his tail,...

There are a couple of main reasons why Theodore Roosevelt couldn't be more aggressive in his first term.

First, he hadn't been elected president but had been William McKinley's vice president.  He had been put there partly because Republican party bosses wanted him out of the way because they didn't really like him.  So in that sense, yes, Congress was on him because they didn't really trust him

Second, TR was the first of the Progressive presidents after a long period in which presidents were not expected to do much.  So it wasn't clear what role the national government and the president should play in the reform/progressive movement.