Being an infantryman in the Roman armies of the kingdom and early republic was not consistently lucrative. To the average Roman citizen farmer there was always a distant hope of booty at the end of a campaign, even in the period you enquire about, but that was dependant upon the wealth of the current enemy. The spoils of war were not uncommon during this period of Roman history, as we are told by Livy in 5.12:
M. Furius in the Faliscan territory and Cnaeus Cornelius in that of
Capenae found no enemy outside his walls; booty was carried off and
the territories were ravaged, the farms and crops being burnt.
With this they advanced by cross marches through the
territory of Caere and surprised the Tarquinians as they were
returning heavily laden with booty.
However, the fact remains that the majority of Roman citizens were effectively obliged to enter into military service if they qualified by falling under one of the first five census classes, a group of citizens which came to be categorised as the adsidui. These citizens were under a legal obligation out of a duty to the state they had a stake in to assemble when ordered by an executive bestowed with imperium as described in 2.27:
Appius was furious; he accused his colleague of courting the favour of
the people, denounced him as a traitor to the commonwealth because he
refused to give sentence where debtors were brought before him, and
moreover he refused to raise troops after the senate had ordered a
And in 5.10:
Then, again, armies had been enrolled for four separate wars in one
levy, and even boys and old men had been torn from their homes.
Earlier in 5.16, it is noted that two consular tribunes, A. Postumius and L. Julius, were prevented from raising the levy by the tribunes of the plebs:
A. Postumius and L. Julius raised a force, not by a regular levy - for
they were obstructed by the tribunes of the plebs - but consisting
mostly of volunteers whom they had induced by strong appeals to come
So even in the exceptional circumstances that a levy was prevented by a legal intervention, there were clearly incentives for Roman citizens to enlist for service in the ranks. Considering this, there would have been those who were willing to join out of the prospect of glory and booty (not too uncommon in a martial society), which may have been in order to attempt to pay back a debt or simply increment their wealth, and those who did it out of patriotic fervor and in defence of their homeland. Perhaps even retribution was an incentive for those who had suffered at the hands of a rival nation.