AWS EC2 Connection closed by when trying ssh into instance

My problem was that the username was incorrect. On ubuntu instances it should be "ubuntu" and on amazon instances it should be "ec2-user".

Often times, you should use the default user name for the AMI that you used to launch your instance:

• For Amazon Linux 2 or the Amazon Linux AMI, the user name is
  ec2-user.

• For a CentOS AMI, the user name is centos.

• For a Debian AMI, the user name is admin or root.

• For a Fedora AMI, the user name is ec2-user or fedora.

• For a RHEL AMI, the user name is ec2-user or root.

• For a SUSE AMI, the user name is ec2-user or root.

• For an Ubuntu AMI, the user name is ubuntu.

  Otherwise, if ec2-user and root don't work, check with the AMI provider.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/connection-prereqs.html#connection-prereqs-get-info-about-instance

Please check your username. if you are using ubuntu instance try using "ubuntu" in username instead of "ec2-user", and vice-versa

Check your security group. Make sure that you have an outbound rule that allows traffic to return from the instance.

With the default outbound rule

type: All traffic, Protocol: All, Ports: All, Destination: 0.0.0.0/0

it will work.