Thomas Goodwin in his book The Heart of Christ in Heaven towards Sinners on Earth says about Hebrews 4:15, "...this text...above any other speaks [Christ's] heart most, and sets out the frame and workings of it towards sinners. [It] takes our hands, and lays them upon Christ's breast, and lets us feel how his heart beats and his affections yearn toward us, even now [as] he is in glory."

Goodwin is pointing out that what Hebrews 4:15 is telling us is that Jesus --- right now, at this very moment, seated at the right hand of the Father --- sympathizes with our weaknesses and if we could literally feel His heart, we would feel a heart of compassion towards us who have placed our faith in Christ.

This is confirmed by what our response is supposed to be. Hebrews 4:16 says, "Let us then with confidence draw near to the throne of grace, that we may receive mercy and find grace to help in time of need." (ESV)

To put it succinctly: Jesus is for His people.

Jesus is for us, His people, because Jesus understands through His own experience of living life in this fallen world what it is like to face testing and temptation. He can sympathize with us. Jesus understands the suffering, the stress, the sorrow of this life. But unlike us, He faced all of it without ever sinning. And this means He can simultaneously sympathize with us while also being the perfect sacrifice for us. His perfect sinlessness did not lessen the suffering, stress, and sorrow; it made it more severe. He never gave in to sin to find relief from it. He resisted it at every turn even to the point of death on a cross.