Why in maximization in OLG model with 1 generations we assume that both consumptions are positive? I have an OLG model with constant level of technology and population. The utility of agent who is born in period t is:

U() = c1,t + 1/1+pC2,t+1

My textbook asks me to write the maximization problem and FOC for the agent  who is born in period t with the assumption that agent can not refuse the consumption in any of the periods. 

I know how to write maximization problem 

U() = c1,t + 1/1+pC2,t+1 -  max c1,t ≥0 С2,t+1 ≥0
c1t + st = wt
c2,t+1 = (1 + r,t+1)st

The only thing which I do not get is why we need the assumtion that agent can not refuse the consumption in any of the periods. What does it give us?

I am sorry of the question is obvious, I am just studying OLG model on my own and do not get everything.
I think all the textbook is trying to say is that there can be no corner solutions to your maximization problem. 

Not refusing consumption in any period, I think translates to consumption in each period > 0.