God is someone we can't design according to our fancies. We need to find out what He is like and then accept it. But we have a tendency to 'like' the 'nice' features of His nature like love and mercy and downplay others like righteousness, justice and impartiality. Yet, how much ever we try, He will remain who He is and will not fit into our mould.
Some people emphasise God's mercy so much that they conclude He can never hurt anyone. They find verses from different parts of the Bible and finally come to the doctrine called universalism, which is essentially that He will finally save everyone and not punish any! What is more common among born again Christians is imagining that there is forgiveness always available even if one goes on sinning because "His mercy endures forever" (Ps.136) and "His mercies are new every morning" (La.3:22,23). These verses are indeed true, but we may misunderstand them.
God's mercy has to be always kept balanced with other aspects like His righteousness, because otherwise we will have a distorted picture of Him. What does "His mercy is upon generation after generation toward those who fear Him" (Lk.1:50) mean? It means that if we want to be sure of receiving mercy from God we must go to Him with respect. "But there is forgiveness with You, that You may be feared" (Ps.130:4). The fact is that if we take His mercy for granted or presume on it and become careless with sin, we can't assume that we will receive it. On the other hand, the Bible makes it clear that for all those go to God in repentance and godly sorrow, God's arms of mercy are always open (Lk.15:20;Jn.6:37).
God warns us many times because He knows that we might just have this tendency to stretch His mercy to cover our careless unfaithfulness. He.10:26 warns against going on sinning. This is about going on doing what we know to be wrong, defiantly or blatantly. Those who are failing even though they are struggling to overcome should not wrongly apply this to themselves and lose hope. But those who plan on asking for forgiveness and then go ahead with sin need to worry.
Those who take mercy for granted need warnings but those who fail while struggling against sin should be comforted. But the former tend to 'rejoice' over proclamations of mercy and the latter tend to lose hope thinking they have gone beyond the scope of mercy.
God's mercy is fresh for today even if we have availed of a lot of it till yesterday, if we go to God in repentance and sorrow, not wanting to sin again. He will offer it tomorrow also. But we would be fooling ourselves if we imagine God is so nice that He won't mind if we enjoy a little bit of the pleasures of sin here and there. It will be painful to discover how 'hard' He can be on those who defy Him (Ro.11:22), and what we have lost in the process. If He chastises us, it will still be from His love, for our good, for us to repent, but it will be painful to go through (He.12:6,11).