To understand this "weird" teaching, one has to realize the relation between demons and our sinful passions. When we are tempted, we are not tempted either by God or by demon, but by our own sinful inclination, that dwells deeply in the hidden and dark recesses of our hearts (cf. James 1:13-14). When we succumb to them, we die with a greater, metaphorical death, than the physical death (cf. James 1:15), for physical death does not deprive us of communion with God (Phil 1:21; or Psalm 63:3, which says "Your Mercy is better than life", for when we die as martyrs witnessing God's Mercy's presence in us, by this we are entering the true and eternal life passing through this physical-biologial death to indestructible divine life of our souls /John 5:24/), while true death is to get deprived of this communion even while physically-bilogically living. And then demons are jubilant, they come and "dine" in our hearts and foment even more strongly our sinful passions so that we may erroneously identify ourselves with them. Now, when Son of God comes to stay in our hearts eternally (John 8:35) together with Father (John 14:10), He defeats the demonic influence in us (Mark 3:27) and heals our sinful inclinations, and our "home" that is to say, our heart, becomes clean and bright, and our mood is jubilant in God, for there can be nothing more desirable and admirable than feeling of a presence and communion with God (cf. Matt 17:4), for such a person smacks of and enjoys the Eternal Kingdom already in this life, for what is the Kingdom of Heaven, but communion with God's Spirit, who also dwells in our hearts with the Father and the Son, giving unspeakable peace and joy (Romans 14:17).
And now comes that dramatic caveat of what could happen next, (for Jesus does not say that it will necessarily happen, but implies that it could or may happen)? If one's heart is cleansed and purified, it is not a static thing, but purification, assimilation to God, admiring a greater and greater intensity of His presence is a dynamic and on-going process! That is why David says: "cleanse me even more, so that I may become whiter than snow" (Psalm 51:7), "whiter than snow" metaphorically implying the saturation by uncreatedness and deification, for there is nothing whiter than snow in the created universe, so the "whiter" implies that the mortal and perishable in us is consumed and clad in immortal and imperishable (1 Cor. 15:53). Yet, this does not happen automatically, and we have to work for it together with God and His grace present in us, for the Heavenly Kingdom is "conquered" only by those who through full concentration of their efforts "force their way into it" (Luke 16:16). But if we are idle and not do it, that is to say, if we do not co-act with the cleansing grace present in us, what then?
And exactly here comes what Jesus warns us about in Luke 11:24-26! We are not growing in God, and since this implies that we start getting indifferent towards the salvific and healthy pleasure of His presence, we start in the depth of our hearts gradually to betray Him, by feeling nostalgia for our sinful pleasures (be it drugs, excessive alcohol or unseemly sexual pleasures etc.), because with total deprivation of any pleasure human heart, which naturally desires for pleasure, cannot live at all! Thus, our hearts incline naturally towards pleasure and happiness. Thus, when God ceases to provide the deifying pleasure to such an apostate heart - not due to God, but due to the sloth of the person himself - then this heart desires even more strongly the sinful pleasures it had refused and abandoned at the time of his conversion; now, the nostalgia for them grows greater and greater, as of somebody who has not seen one's homeland for a long time. Eventually, this nostalgia brings back an apostate heart, as to a homeland, to the same sinful passions from which it had departed, but now with a novel impetus, novel and greater strength, endearment and passion! So, such a heart embraces the previous sweetness of sins with a greater thirst and a greater thrust. Thus, the condition of such heart will be more miserable, because the committed sin engenders death (James 1:15), and deeper the sin - deeper the death. This greater depravity of heart means that the demonic presence in it will also become more intense and more difficult, if not next to impossible to get rid of it. This is the meaning of the metaphor "7 more demons" in fact. Yet, "next to impossible" or even "impossible" for men is possible through loving God (Luke 18:27), for gates of repentance are always open.