To be eligible to run for the United States senate, you have to be 30 years old, a U.S. citizen for nine years, and residents of the state they're running in.
The Constitution requires the Senate to ratify treaties. Until the Senate approves them, they are not in force. The Senate does not automatically approve all treaties that the President sends to them.
Because the constitution requires it
A joint resolution is a legislative measure that requires approval by the Senate. It also requires approval from the House of Representatives and then requires the Presidents approval.
The U.S. Constitution requires that the Senate have two officers. Theses are the President of the Senate, which is the Vice President of the United States, and the President pro tempore, which is traditionally the senior member of the majority party in the Senate.
Because the constitution requires it
power to make treaties
requires a 51 of 100 (majority) vote by the senate
The appointment of executive branch official must approved by the Senate. These include cabinet appointments and ambassadors. The US Constitution requires this interaction.
Treaty
There are 100 representatives in the Senate as the United States has 50 states, and therefore, there are two representatives in the senate representing each state.
The Senate and House of Representatives are government institutions which can initiate the impeachment of the president. This is an extreme measure which requires complete backing by both.
It requires 60 votes to shut off debate in the Senate. Debate can go on indefinitely (called a fillibuster) unless the Senate votes to shut it off . A bill cannot be passed until debate ends and a vote is taken.