Why did the north feel it was unnecessary for secession?

The north never had any intention of secession. The reason the south seceded was because they wanted to separate themselves from the northern states and territories in order to pursue a government that they thought was more appropriate (more emphasis on states' rights and approval of slavery). The north's goal during the war was to keep unity in the country and try to get the southern states back in the Union. Secession of any northern states would have been the exact opposite of what they were aiming for.