There's a lot I could say about this, but I'll try to focus and keep it brief.

TL:DR Men enlisted for a range of reasons; they signed on for the adventure, to get away from home, to stick with their friends, family and existing militia companies, and to be part of something 'big' and 'exciting'. In 1861 in particular, few had any accurate sense of what the war would entail, and how deadly it would be, and thus the risk of death was scarcely conceived; thus, men didn't think of the risks as all that great [this changes later in the war]. Newspapers certainly tried to whip-up a frenzy of anti-South sentiment, and that frenzy certainly fed into those broader sentiments, but it was often a mix of personal circumstances, and there was rarely one single reason for wanting to fight. These reasons for enlisting changed as the war progressed, but this was the case in early 1861 when war broke out. .

To give some background, in 1861, many of the initial recruits who fought for the north came from existing state-oriented militia companies. Those companies served as community organisations, and gave young men a social group to identify with. Companies were often formed around villages, or even workplaces. For example, according to Stanley and Hall, of the 2nd Maine, which formed in 1861, the militia men of Company A consisted mainly of a ‘class of men, that would, in these days of quaint expressions, be termed the "top knots" of the town, or "tony”’.[1] Company B – Castine Light Infantry, Company D – Milo Artillery, and Company K – Old Town Company, were formed around particular towns and localities near Bangor, while Company G – Ex-Tigers & Amory Associates, consisted primarily of former fire-fighters from Bangor.[1] As Wise noted, 'In each case, the men who formed these companies had strong civilian connections with each other. They were neighbours, friends and workmates, who spent much time together in civil society. As they joined the military together as a group, those common connections helped reinforce their views of military service' [see Wise, The Pursuit of Justice, pp. 41]. Discipline was lax, regimental officers were often political appointments, while the rank and file typically voted for their company officers, and most men identified as 'citizen-soldiers' who approached 'service' as a casual affair [see Wise, The Pursuit of Justice, pp. 32-40].

Albion Tourgée wrote a lot about the cultures of these units in his book, The Story of a Thousand. There's much of relevance I could quote from this, but this highlights a core aspect of that culture:

'The men these officers commanded had been their neighbors, schoolmates, friends. No wall of exclusion separated them; rank made little difference in their relations. They found it not difficult to command, for the only deference they exacted was the formal one their position required'.[2]

When war broke out, those existing companies were the first called on to help form regiments, and the men of the companies were asked to volunteer for active service with the North. Many feared the war would be over before they could participate, and so they were keen to sign whatever they needed to get going. John Gould, who served in several Maine regiments, wrote,

'Few of us stopped to ask questions, for at that day the only idea that possessed the public mind was, that the North would ‘rise in its might’ and squelch treason in a hurry, and you remember, my friends of the ‘1st’, our constant fear was that we should never get away from Maine till after the N. Y. and Mass. Militia had suppressed the rebellion.' [3]

The idea at the time was that those militia companies were state-based, organised principally around state-defence, and thus they were asked to sign on for Federal service, initially for 'three months from date' [Wise, p. 43]. At the time, three months of service didn't seem like much to those men, and many eagerly joined with the rest of their company.

Consideration of the South as an enemy worth fighting certainly factors into those decision making processes, and this was exacerbated by early conflict. Wise noted that,

'the 6th Massachusetts Militia were reportedly engaged in fighting against Confederate sympathizers and anti-war Democrats in the streets of Baltimore, and a large Confederate army threatened Washington. In the minds of many, the rebellion had to be crushed, and the safety and security of the north preserved.'[4]

In the quote from Gould above he noted 'squelch treason in a hurry', and other men expressed similiar sentiments. This was also widely reported in the press too.

So, those initial recruits of 1861 signed on in a rush, to stick with their friends, family and company, to join and participate in the war before it was over, and to fight 'treason' and 'treachery' from the South as had been reported in the local press.

As the war progressed, the reasons for enlisting changed, and the motivations a soldier gave for enlisting in early 1861 could be vastly different to the reasons a soldier gave in late 1861, and, they change again from 1862 and so on.

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[1] Stanley and Hall, Eastern Maine and The Rebellion, p. 33.

[2] Tourgée, The Story of a Thousand, p. 43.

[3] Gould, History of the First - Tenth - Twenty-ninth Maine regiment, pp. 19-20.

[4] Wise, The Pursuit of Justice, p. 42.